Most people gave very little idea of the exact degrees to which alcohol affects them. If you are a heavy drinker, a glass of wine at her place probably won't affect your judgement or ability. Personally I only accept it if it is her wine, but few are that generous.
As to benefits vs drawbacks, if you can guage the amount and onset period it shouldn't be hard to work out. A mouthful of vodka just before a ten minute taxi ride to her place for instance. Treat the punt as the pleasure and the alcohol as a carefully imbibed drug.