Most LED's, 12v will fry them in a second without a resistor. Hooking them up in series, you can only really hook up 3-4 at a time depending on their voltage usage (for series, you add them together 3v+3v+3v+3v=12v) Mind you, that is per set. You could have multiple sets running. I would go for the 5v power. Since most LED's are 3.3V-3.5V, its a lot less of a step down from 5v to ~3v than from 12V. I either use the 5V line and do each light as a seperate line with a seperate resistor or wire them in parallel although I prefer the first way. That way, each light has it's own resistor and can also be changed or worked on easily.
for a resistor calculator that may explain some of it. But the resistors, if you're already ordering lights, they are like 5 cents a piece and will get your lights the exact voltage they need.