To elaborate on this,
There would not be any noticable difference between a 3.6v and 3.7v and some people make mods that go up to 5v or higher.
I have seen the 18650 batteries go for as long as 10 days with no charge, so they are good for some applications. Their biggest draw back is their size.
If you are looking to make something really portable, you might consider experimenting with cell phone batteries. You want to get something with at least 3.7v on up to 5v (most cell phone batt are 3.7) and as high Mah as possible. The Mah will determine how long the life will be on a charge, the higher the number the longer it lasts.
I have all my mods being build with mini usb ports on them for charging, which gives me some nice options on what I can use (any blackberry charger etc) Add that to something with a slim case and a cell phone battery and you have a nice mod that will be usable while you charge it and last at least a full day on a charge.
I will have some really nice wooden box mods available soon for anyone who is interested. They will all have the 18650 battery (2400 mah) and a mini usb charging port. Can be made to fit 510 or 808 depending on what you like. For now all will be 808.