Licking ass can be amazing for both the giver and the receiver. The key is just making sure the area is washed with soap and water before its performed. That will remove all fecal matter where your tongue is going so you don't have to worry about E Coli. I've never done it knowing the area wasn't washed. It's definitely a common act, and I personally will do it to every single woman I hook up with (for the most part) if she is up for it because it's fucking awesome. And once I've done it to them, they've always gotten so worked up that they will do it right back. It takes foreplay to a whole new level and makes the whole experience more raunchy and wild than the usual routine.