If it's a long ride, I'll cut it off at 30, if it's just to/from work/wherever, I'll cut it off at 40ish, depending on the direction. If it's blowing hard from the west, then it blows me back up the hill to my house! if it's opposite, then it's much lower, like 25.

Something I discovered for really bad cross winds: hug your top tube with your knees and loosen your upper body; then you'll be more stable.