I am one of those chorus teachers who recommend avoiding dairy products before singing. It's strictly from my own experience- I try to take only clear liquids because I personally am very affected by dairy. I once did a straw poll of my chorus members and some said dairy seemed to have little-to-no affect, others agreed that dairy was a problem for them.

I don't drink alcohol, but when I'm singing (at rehearsals, before a concert or sing-around, etc.) I carry a water bottle with either a lemon wedge or slice of fresh ginger, because I happen to like how it tastes and it seems to help my voice. Honey is good, too. Room-temperature works best for me.

If you are singing regularly, I guess the thing to do is to find out what works for you.