I'm sure there is nothing more wonderful then watching your baby being born however I believe the man is there to support and comfort his partner throughout the labour.
If your partner wants you down there then great, but I think in most cases a woman would want her man up top to encourage her, hold her hand, cool here head, and let her know she's doing great. He is there to take her mind off what is happening down there.
This!
I don't think this is a decision you can make beforehand. You are there to support her and unless she chooses to hang off a midwife or bed etc, you just may not see the head crown and be born because of the position she chooses to be in.