There is a fundamental question about lithium rechargeables of any chemistry (Li-ion, Li-Mn, Li-FePo4, Li-Po), which is:
"Should I use the battery until it is dead, or should I recharge it before it needs it?"
The answer is simple:
Recharge it early, and don't charge it to the max, because that will prolong its life by more than double, according to manufacturer's tests.
But: if this is more hassle than you need, and/or you can afford to replace batteries more often and just don't care - then simply don't bother.
The voltage it is charged to is as important as the discharge voltage. So if you want to get, say, eight hours out of a lithium cell, then by all means charge it to the max and discharge it till it's flat. But if you are OK with getting six hours from the battery, and want to save money: then don't charge or discharge it fully - and you'll get double the number of recharges from it.
Charge it to 4.17 volts maximum, and discharge it to 3.4 volts minimum.
There are many reasons for this, and some are listed below.
Manufacturer's tests for Li-Mn cells like the 18650 show that if the cell is charged to the max (4.21 volts) and discharged to a normative minimum low point (a matter of debate, but somewhere between 3 volts and 3.3 volts ), then you may get around 200 cycles. In other words you will get about 200 recharges before the cell won't hold a charge and needs disposing of.
If the cell is not charged to the max but only to about 4.17 volts, and not fully discharged but only taken down to about 3.4 volts, then you may get over 400 cycles.
Let's assume that these service life figures are absolute max and min numbers and in real life they may not be achieved; even so there are clear advantages to not fully charging or discharging a lithium cell. You get maybe only 75% of the charge, each time, but you may double the battery's service life. It allows you to choose which you consider most valuable: the longest possible life for the battery, versus a maximum charge each time.
Why charge/discharge it to the max?
Well, it could be that you travel a lot and charging facilities are difficult, and you don't like to carry too many spare batteries, so each one needs to last as long as possible.
Or, perhaps you don't want the extra hassle of worrying about any of this, and you don't care about the cost of buying new batteries maybe twice as often as necessary. You've got a box of ten of them so it doesn't matter.
Why take extra care then?
Your batteries will last longer and will cost you less. And, it's greener
However, each charge will be be about 25% less than the max, so a battery needs charging more often.
Why do some people say batteries need a full charge or discharge?
This is left over from the days of Ni-Cad batteries, when they were the only rechargeables available. These cells had a memory effect and if you didn't fully recharge them, they would 'learn' the reduced charge level and not accept a full charge; and since, over time, the charge accepted also reduces, it leads to early battery death.
This is all over and done with now, lithium cells have no memory. It has been suggested that it may be better to 'fully-charge' a lithium cell the first time it is used, but since the best chargers don't go past 4.18 volts, this would be impossible anyway.
It might also be worth considering that the concepts of 'fully charged' or 'fully discharged' don't really exist for batteries: what you have is a voltage range at the top end where reduced service life becomes more prevalent the higher the charge voltage is, and the charge capacity also becomes affected; and the same at the bottom end for discharge voltages, where the lower the voltage goes, degradation increases. All you are really doing is agreeing to voltages that are a fair balance between performance vs service life. Going beyond a certain value, at either end, leads to degraded performance; and there is a value at which the amount of damage increases rapidly.
How to tell the discharge level?
There is a problem with many APVs in that you can't tell the battery discharge state, i.e. when to recharge it. You can either pull it out and check it with a meter, or make sure to recharge your cells regularly even though the APV still works well. Protected Li-ion cells are OK as they cut out when too low, but it is a problem with other types like Li-Mn (AW IMRs for example).
This is a problem because running a lithium cell too low will damage it; and approaching anywhere near the damage point reduces the battery service life. Ideally every APV, even the basic electrical type, would have a battery voltage readout or a protection circuit.
My charger goes to 4.21 volts, what can I do?
You will get the absolute maximum safe charge for the battery. However, the service life sees some reduction: it won't last as long. There's nothing you can do about it, except remove the battery before it is fully charged - let it rest for 2 minutes - measure the voltage - decide whether to leave it, or put it back in for more charge (if it's below say 4.15 volts).
To get maximum battery life you need a charger that doesn't go above 4.17 volts. To get a maximum charge each time, without worrying how long the battery will last, you need a charger that goes to 4.21 volts. To be able to choose between the two, as required, you need a charger with a variable voltage cut-out at the top end. There isn't one - a gap in the market, perhaps, for a charger that has a dial variable between 4.16 and 4.21 volts, and an LCD voltage readout.
 The concept of a 'minimum low point' is a flexible one. An Li-Mn cell should not be taken below 2.5 volts as that will lead to damage, that is to say, an immediate reduction in capacity and recharge cycles (service life). If it is always taken below a point somewhere between 3v and 3.3v (the exact figure is not agreed) then some reduction in cycles will be seen. Essentially, the higher the voltage when a decision to recharge the cell is made, the longer the service life of the battery, and 3.4 volts is probably an optimum figure at which to recharge the cell if maximum service life is required and an early recharge can be accepted (it is always a balance between the two).
The other end of the scale is easier to define as we know that 4.17 volts is the optimum balance between service life and a single charge's capacity.