It's true that if you record quieter, you're not using all the bits available... but it just doesn't matter.
Every additional bit added doubles to possible amplitude you can record, that is, it increases the dynamic range by 6 dB. I don't want to get to involved in the way people perceive sound, but if you're interested just leave a comment and I'll be happy to explain further.
Anyway, CD's are encoded in 16 bit. So if you're recording at 24 bit - and you should be - that means at least 24 dB of dynamic range are going to be dropped when you finally encode your music onto a CD. So don't worry about getting those input levels so hot, it opens the door for lots of other problems, like distorting plug-ins that aren't equipped to handle such hot input. Distorting plug-ins aren't usually the sound you're going for.
I don't want you to think I'm saying you should record really quiet either, just leave yourself 8 to 10 dB of headroom; you should be able to see the waveform whenever you hear sound, but on the other hand, it shouldn't ever look like it's in danger of touching the edges of the track (clipping).
If you want to be loud, you can always throw a mastering limiter on the output bus and boost the levels up to 0 dB there.
edit: I accidentally switched up 3dB and 6dB at the doubling of amplitude part of the post. A Doubling is power is 3db. A doubling in amplitude is 6dB... sorry about that. It's fixed now :)
correct me if I'm wrong, but I believe this is because doubling in amplitude doubles the size of the waveform in both directions (up and down from 0dB)