The answer to this question is way too broad for a straight answer. It is going to end up being made up of biased opinions because several conditions and or anomalies can occur affecting ones decision on whether or not to join a bounty campaign at any given time (Before ICO/IEO or During). It could be easier to join a bounty campaign during the initial offering sale to the public, this way if the campaign is proving to be successful, there is a chance the campaign could be extended. Instead if you go the Pre-ICO/CEO route which is before the fundraising, you give your self time to earn from a project and start your planning at hunting for a new campaign after the one you joined early finishes!
  As I said, the reason on which someone may join at this particular time, or that particular time is going to be different for every individual, so to answer your question, just simply join a bounty campaign during a time that is most convenient for you!