For me, back or legs are completely out of the question with a hangover. Just the thought of that makes me want to puke.
When I was in college I worked out with a hangover more times than I could count and still made progress. I had days where I sweated out so much bourbon that my buddies could smell beam from across the room.
There's no doubt I could have made more progress if I had lived sober, but wheres the fun in that? Since I graduated I rarely slam booze and have made the best gains since i started lifting, so there is absolutely a correlation between alcohol consumption and gains.
To answer your question, I think its better to plow through the workout than take the day off when your hungover. Completing a workout when training is the absolute last thing that you want to do is what separates us from everyone else.
Physically, the hungover workout is most likely not as effective as a normal workout, but the mental achievement and pride that comes with remaining dedicated and working towards your goals no matter the cost is more beneficial than taking the day off to lick your wounds.