I certainly don't pump up before every ride. I do regularly check the pressure (thumb test), but if it feels OK, I'll ride. Maybe I'll pump the tires once a month or so, or before a big ride (100+ miles, or a few hundred miles RT). It is also tire size dependent. My 20x4 1/4 tire can hold a lot of air.

As far as bleeding flat.

• If the tires become low (still some air inside) over a few weeks, it is probably normal air leakage due to disuse.
• If the tires become completely flat, beads unseated, no pressure, then it is often a sign of a slow leak (puncture or faulty valve?) unless the tire has been sitting for several months, or perhaps over a year.