I pump by bike tires up before EVERY ride; for road tires, I find that I lose a little bit of pressure after every ride; it is imperative to have a good pump with that measures above 150 psi; depending on what tire you have, you can pump up tires up to 160 psi IIRC.

I pump my Fortezza's up to 140 psi before EVERY ride; most rides I do are 50+ miles and if I don't pump them up, I can definitely tell a difference. It is a good idea to check the reading for PSI on the side of the tire you are using and go from there.