I would only post such a comment given two circumstances:
You can clearly see the asker of the question is a new user (or hasn't accepted an answer for any of his questions), and might be not aware of the positive effects (not only to the answerer, but to the whole community) of accepting an answer to his question. If it's not the case, it's clear that if the OP hasn't accepted the answer, it isn't because he didn't know he should.
Enough time has passed / the OP left a comment explaining how useful the answer was, so you can be positive the answer has been really helpful and the OP isn't waiting for a better / more complete answer.
In the comment I wouldn't just ask to accept, I'd rather link to How does accepting an answer work? and point out the benefits it will have for future visitors of the question to have a clearly accepted answer.
In the end it's not about you getting rep for the answer, or being a rep-whore, it's about educating new users.