I know, I know, it's really up to me, everyone's portfolio is different, but maybe my situation has a slight twist to it...even though my situation is small potatoes...I hope to be a bigger spud someday! 
 I have about 450 pics up on iStock, 241 downloads.  I suspect I will be eligible by the end of June, the way things are going.  Despite all the negatives voiced on this site, iStock is easily my best agency; 4 payouts in 2.5 years.
451 pics on Dreamstime about 260 downloads.  I tried exclusivity on Dreamstime for a bit but then the downloads dried up and went back to Indie.  Haven't uploaded anything in a year so that I could deactivate my portfolio when the time came. 3 payouts in 4 years.  I only had 3 downloads in May so am definitely not holding my breath on Dreamstime
218 pics on Shutterstock and have only made a payout once in 3 years, ~$65 in the bank after a large sale in last year of $14. And it's not for lack of trying to upload more, just kept getting the rejections.

I am definitely going to give iStock exclusiveness a try but I wonder about the timing of it.  Should I wait until I reach a payout on Shutterstock?
Another question I have always had: Is it worth it?  Will I reach a payout per month+ in the foreseeable future?  The enticement page says that I will see a difference in the number of downloads...Did those of you who are now exclusive / went exclusive with a similar size portfolio see a jump?