Does anyone tip a stewardess for beer or wine purchace in coach?

No - one does not tip FLIGHT ATTENDANTS. (the word stewardess is not appropriate - and the one serving you is just as likely to be a male)

I have always tipped them and I have seen others around me do the same.

I have a friend whose wife is a flight attendant with Delta. I asked the same question, and the answer was, "no, we don't expect tips." Her reasoning was that unlike waitresses, they don't make sub-minimum wage, and certainly don't rely on tips to make a living. Their number one priority is passenger safety, not serving passengers beer and wine.

Maybe this varies per airline or flight attendant, but she stated that she didn't know any flight attendant at Delta that expected to be tipped. She said it was almost embarrassing when passengers tried to tip her.

I always have too. It is nothing much but they do remember me for the rest of the flight so it makes those over sea ones not so bad.

Yeah-but then you have to tip the pilot and then on and on ... by the time you get off the plane, you are broke!

My ex-sister-in-law used to be a flight attendent for Delta. On a Christmas Eve flight to Caracas, one man gave out $100 tips to each flight attendent. I think there were about 7 of them. This was about 30 years ago. It was her favorite night in the airline business.
I seldom buy drinks on a plane, but I have often seen most people around me give the flight attendent an extra dollar. I've never seen a flight attendent surprised or hesitant to take it.