Li-Ion batteries can be safely charged up to 2C (two times the rated capacity of the battery), but ideally should be charged at 1C or lower. So if you go below the rated capacity of the battery, then you will be perfectly fine, it just may take a while to charge the battery.
For example, say you have a 900mAh battery and a charger which outputs 4.2v @ 150mA. Well, it will take you roughly 6 hours to charge the fully depleted 900mAh battery, but it will not be dangerous (unless it trickle charges and doesn't terminate the charge once the battery reaches around 4.15 - 4.2v charge, which is a big no-no for any Li-Ion charger).
Now say you have a charger that has an output of 4.2v @ 1000mA and you are trying to charge a 150mAh battery, well that is VERY bad and should NEVER be done.
A good rule of thumb is simply sticking to 1C or lower. If you battery has 900mAh, then as long as the charger has 900mA, or less, charge rate @ 4.2v, then you will be fine. This is also a good idea since many Li-Ion manufacturers exaggerate their mAh ratings (*cough*Ultrafire*cough*Trustfire*cough*) . For example, many 900mAh 14500 Ultrafire/Trustfire batteries are actually between 500-750mAh. Only a few Li-Ion suppliers/manufacturers actually state the real numbers, like AW for example.