Yes.
Being best man is very stressful and i was loaded pre-wedding. Managed the service ok (just had a couple of pints to top up in the morning) then got worried about the speech and started drinking the complimentary wine and had a few more beers. More wine with the dinner, worrying about the speech, and a crafty few shorts. I'm told the speech went "ok".
If you're just a guest nobody will notice if you're a bit "merry". Just go with it and talk to lots of people...talking to strangers is always better when you're a little smashed. Exciting things happen which wouldn't occur if you were sober. And you'll almost always get a great anecdote to tell at the next wedding you get invited to.