With the right guy, I freaking love getting rimmed as well as rimming. But it's not for everyone, either role, so it's not necessary for me, just a bonus.
Although I really enjoy receiving, I won't usually ask for it - it's hotter when he initiates it, because then I know he enjoys giving, and usually he's better at it because of it. The guys who tend to like it are either tops who just love ass (one guy I used to mess around with would just park his face between my legs with me on my stomach and go to town until I was ready to move onto other stuff - he really got into it) or the sorts who just like to give oral pleasure, from licking nipples to sucking cock to eating ass, they want to get their mouth all over, heh (and it's appreciated!). Since I'm a shorter guy, 5'8", I've noticed when hooking up with 6-foot-plus guys that a hot 69 involves me on top giving head while he rims me since it's easier to reach for him. Good times. I tend to be pretty fastidious about keeping my business clean, but if it's a surprise hookup and I have any doubts at all, I won't let it go there - it's courtesy, heh.
I don't really get motivated to rim a guy unless he has a nice ass (and if he seems to be on top of his personal hygiene). If he does, though, watch out - I dated a former gymnast who had the most ridiculously full, rounded ass that was the perfect mix of muscle firmness (with awesome dimples) and bouncy bubble, and man I used to love burying my face in that.
Some guys can get a bit weirded out if they're not into it or expecting it, so I usually ask first - but I've also had guys flip over while I'm giving head and wiggle their butt like they want it, and I can take the hint, haha. If I already know he likes it, I'll also just lift his legs and move down from sucking to rimming, too. If I kind of get the sense that he's not completely clean (but enough so to work with) I usually use some saliva to help and rub the area sensually first - no need to embarrass him; and if after I start I can tell it's just not a good time for it, I'll just tough it out for at least a little bit longer and then move on to something else so it's not so abrupt. I'm proud to say I've given several guys a great first rimming experience - most of them became converts right away, ha.