Should I leave some headroom?
Yes. It doesn’t have to be much – preferences vary from engineer to engineer – but usually anywhere from 1 to 6 db seems to be common. You don’t want to be slamming your meter at just-below-clipping 0dbfs, because it gives the mastering engineer a little less breathing room (and it also means there’s a higher chance of “intersample clipping” – see below). In the old days, you didn’t want too much, because everyone recorded at 16bit, and too much headroom meant a much lower signal-to-noise ratio, but if you’re recording at 24 bit you’ve got something like 48db of headroom before you’re risking signal degradation.