For me it depends on the weather. I keep my hair buzzed pretty short, and on cooler race days I like the cap to keep my head a bit warmer. If I put the bill in the front, I find it hard to see, and I can never keep it flipped up. So I usually wear it backwards.
If it's raining, I'll definitely wear it with the bill in front, to help block some of the rain. But it also blocks some of my vision, too, when I'm in the drops. Does anyone else have this issue? How have you resolved it?