Is a cash bar for our wedding tacky?

We plan on having a cash bar at our wedding in September and I Googled how much we will need to purchase (we have to purchase the alcohol up front, but then the guests will be charged at the wedding to cover the cost of the alcohol). The more research I did, the more I realized a lot of people were saying its tacky and rude to charge for the drinks. Here is what we were planning on doing. We were going to provide ALL pop, water and 2 kegs of beer for our guests and only charge for the wine, champaigne and mixed drinks. I've heard of that being done at many weddings. I was in two different weddings last year and they both did the same thing. I didn't view it as tacky in the least. Opinions?