In my case, I set my all gigs as promoted and get some stats for 2 to 3 days, and when I get to know which is performing well, I set the remaining as not promoted. May be this perception might change as I am getting more stats and details.
Note: I'll upload details again after few days for my latest stats and details of promoted gigs results in community here again.
and yes I set to auto (Even though a lot sellers also suggesting to set it to auto).