Flagging doesnt hurt you at all. It is simply a downvote and removes the amount you downvote equal to that you would have upvoted.

For example, If your 10% upvote is worth 35 cents then a 10% downvote would remove 35 cents from a post.

Also if you have a higher rep than the person you will remove a little rep of theirs too

One warning however, it can hurt you as a side effect if the person you flag gets pissed and starts flagging you back