I usually have a symmetric placement of events (although not always exactly the same weapon type). Second to that is a mostly symmetric placement of masks (meaning platforms, corridors etc.) so that the ammo isn't just spread equally, but that it is also equally accessible. I usually vary a lot with platforms, but for corridors and more "solid" ground/mask parts, I stick to a rather strict symmetric schedule (see KMC, NiN). It's because I do believe that symmetry is important and "fairer" indeed.

To the named examples: in BBLair, I always preferred to play as blue actually because I liked to have the seeker ammo (20 from repeated powerup getting) close to me. However, that was in the old days when duels there ran a lot different (that sounds stupid but it's just like that), e.g. many people didn't know about blasting the red carrot down with TNTs etc. etc.
Semi: I think semi sucks anyway (I guess that's not a secret :P) and I totally suck there myself, no matter which side. If I'd have to decide for which side I hate playing less, I'd definitely go for red though. Easier navigating around your base, more immediate access to the seeker powerup from a position close to your base (don't have a two seconds delay through the tunnel) and such stuff.