There's no general answer to that, because each campaign manager has his own standards.  Basically if the manager cares one whit about this forum's quality, he won't choose people who consistently make garbage posts.  Unfortunately there are managers who don't care, and they have to fill up the campaign with participants--the result is that you get very low-quality posters making a huge amount of nonsensical crap posts. 

The bitcoin-paying campaigns are generally run by good managers like Yahoo62278 and DarkStar_ (though they aren't the only good ones), while altcoin/token-paying bounties tend to be run by managers with low standards.  At least that's my observation.

Rank does matter a lot, because higher ranked members have more of a signature space to advertise in.  The good thing is that Theymos created the merit system, which made it very hard to rank up.  That doesn't discourage Jr. Members and the like from participating in campaigns, but it does limit their earnings if they suck at post quality.