I usually have that glass of wine on flights to Europe, or if I'm in first class (now that DH doesn't fly so much, not very often).
I don't usually drink on my trips to Phx and San Diego, but it doesn't mess me up to have one drink, any more than if I had it at home!
I just can't be perfect all of the time and worry about dehydration, having to pee, etc. I will get up to go if the seatbelt sign is on... really, I've never been stopped. I wouldn't do it in turbulence, but other times, yes.