0dBVU (normal, everyday, line level, "where the pros record," how they do it "downtown" etc.) is around -18dBFS. If you keep the meat of the signal around there, you almost can't go wrong. In 24-bit, there is absolutely no reason to go higher. In 16-bit, it's arguable. Personally, with decent converters, I might go just a *whisker* higher (but then again, I wouldn't record in 16-bit anyway).
This is how the gear is designed to work. Recording at -6 or -3dBFS is going WAY into the headroom of the preamps. You lose clarity, you lose focus, you add distortion and noise, you "pinch" the image, it makes the tracks harder to EQ later - All sorts of nastiness comes from tracking too hot.
Plus, after you track too hot, you have to turn all of those distorted, fuzzy, unclear tracks DOWN to a decent level (go figure, it'll probably be around -18 to -20dBFS) to mix. Seems like a waste, no? Or, you can start with a bunch of tracks that were recorded at the proper levels, and the mix almost takes care of itself. There's a reason for that...