Good question. On giving this a mulling over, I estimate that I say "Excuse me," about 33% of the times I sneeze.
I think the main situation where I would say it would be if I was with people I know and there was some funny/goofy way to say it. I don't actually consider it appropriate to say "Excuse me," about a sneeze. Excuse me suggests I have done something wrong or something impolite. A sneeze is completely involuntary and is as big an imposition to me as anyone. It makes about as much sense for someone else to say Sorry when I sneeze as for me to say Excuse me.
I may be wrong about that. Maybe I say Excuse me more habitually. I'll have to monitor that going forward.