Every ride, no matter the tube material. Checking often can alert you to slow leaks too that may become faster ones at the least opportune time.

Pre-ride checklist:

1) Check tire pressure.
2) Check brake quick releases (never know when one might have been monkeying with them).
3) Check brake pads (never know when you've picked up some crap in the pads).
4) Check seatpost height (the one I have right now slips every so often).
5) Check wheel quick releases.
6) Check wheel brake surfaces for dings (hate to grind away brake pad unnecessarily).
7) Check seat bag for all the "stuff".

I do pretty much all these before I ride, or before I load up to drive to a group ride (yeah, I know, but necessary sometimes). Takes about 3 minutes.