Granted, I'm no bartender but, I do know a few things from doing cocktails.
At the casinos, etiquette is 20% or a little more if your barback is doing
barback duties ONLY. If s/he is consistently serving, taking orders, and
building rapport with your clientele because you are busy, Vegas etiquette
would be to pull with them (or with the one who is busting butt to make money
for YOU).
I saw in 2Rude4MyOwnGood's reply to tip 12-15% if they do barback duties ONLY.
Like I mentioned before, 20% or a little more is our threshold because out
here when you tip in that range, "word gets around" and your barbacks may not
be there to help you out. Think about it, they're working for tips just like
you and I.