i have this belief that i only charge phones when it's empty or less than 5% to prolong the battery life...

is there a way to connect the phone to the pc w/o charging it?

Hi.
N900 uses a single cell battery with out memory. Therefore, you shouldn't go under 10% or over 90% too often. It prefers being charged more often, for less time!
You should charge it whenever you can, keeping it between 10% and 90%