I generally keep my gaze straight ahead when I'm riding (or glancing at the ground, keeping on the lookout for broken glass -- grrrr!), but that's mostly because I'm kind of shy. If another biker waves at me, I sometimes don't notice or I notice too late to wave back! Many times people who come up on my left and pass me say a friendly, "Good morning!" or "Hello!" -- this sometimes catches me by surprise, and I'm usually late in getting out a weak reply. I have to say it's nice, though, when other riders are friendly. I don't mean to appear unfriendly -- I'll try not to be so uptight!