I think the best advice ever is attend the ceremony, make a brief appearance at the reception if you feel you must, then high tail it out of there..definitely no later than the bride and groom, and preferably a quick exit well before that.
You'd have to go home if you were feeling unwell for example, or any one of a hundred other valid reasons, so it always baffles me a little why people see this differently to be honest
If you must go this is a pretty good link of stuff to consider:
http://www.soberrecovery.com/forums/...val-guide.html
D