Both "acceptable" and "accepted" are correct in your sentence.
"Acceptable" means something is considered good enough; it's not objectionable.
"Accepted" means that something is considered right or suitable by most people.
I think, though, that your sentence would be better without either "acceptable" or "accepted." It would be perfectly understood if you said, "It is the custom for one to say ˜excuse me' when he sneezes."
Rachel