If I can't lean the top of the headstock against the wall (or whatever) then it leans backward. Reason: I would never lean against the fretboard itself. (Although nice, soft couch cushions wouldn't bother me or it.)

If it's against a wall, then it's leaning forward against the headstock. Reason: I never see an old neck bowed backward (away from the fretboard), so I always think to counter the string tension. (And it's true about the tuning pegs, too.)

If it is on a stand, then the neck support is lowered as far as I can get it for the same string tension reasons.