I would say that bounties can affect your reputation and number of Merits that you will get. Some people before awarding user with Merit check his post history. If he will see lot of bounty reports in your post history, it's likely that he wont give Merit. If you don't want to be callet Twitbook spamner, before joining social media campaign be sure that reports should be made in Google forms.
As mdayonliner said, participation in signature campaigns shouldn't affect your reputation. If you are making good posts, you will receive Merit as every normal user of Bitcointalk. No matter you wear signature or no. Just be sure that you don't advertise scams or something illegal in your signature, otherwise you will get big red tag under your username.