I know that if your server bursts a lot you could pay a lot of money for 95% billing... but if you have a server w/ 300 gigs of bw being billing 95% usage and the sites you have just use a steady amount of bw every month, is it possible to get a full 300 gigs of usage and not be billed any extra?

Theoretically, yes. Practically, no.

Of course since you'll typically see $/GB measured at 95th percentile rate being much lower than $/GB measured on total transfer, you'll often do better buying at 95th percentile.

It's a simple math game:

Determine how much actual bandwidth your server uses.

Determine how much bandwidth your server uses measured by the 95 percentile method.

Then look at the prices for buying actual bandwidth, and look at the prices for buying 95 percentile bandwidth. Do the multiplication and see which is cheaper.

Like cperciva says, because companies that sell bandwidth by 95 percentile tend to charge less $/GB, if your traffic pattern is pretty steady (rather than bursty), 95 percentile billing would probably end up being cheaper for you.