Sometimes it's all about timing... When you grab her ass, and what kind of mood she's already in.
If you grab it in the kitchen...When she isn't expecting it, and aren't even lookin to get laid... just grabbed it because "it looked so nice... I just had to grab it" and then walk away.. Like into the other room.
If you guys have been dating for a short time, I don't think its a good idea. But if you guys are in a point where its all confortable, then there's no need to worry. I personally think that sometimes it makes me think he is only for sex. but again it depends on the level of your relationship.
well I have a girlfriend and I usually grab her butt, then after our date or at the end of the day I ask her bout it and if she says she doesn't mind then go do that alll the heck you want just don't do it in public too much cause it might be a turn off to her if people are watching this going on