My opinion is to get into the top agencies (my big5: IS,SS,DT,FT,SX) and just forget about the rest. You could try to add 123rf, canstockphoto and crestock just because the uploading process is so easy, but generally speaking the rest of the sites gives just peanuts and it would be better to spend the time shooting new stuff.

personally it depends on your portfolio size.  if it is small (unless very special then I'd work on the top 6-8.  If or once it is bigger then add more.  Most of the lower ones have very easy uploading so arent a big deal to in this regard and you can go back and upload your entire port.  But each is at best 1-2% of income.

Basically if your earning say $100 a month overall,  adding 4 more lower ones will then add aroudn 4-5% or $5 for the month between the sites so it will be a long time before payout. But at $1000 a month, your adding around $50 with one paying out each month and so on, to me thats when it gets worth it, others may say a $1000 week or not worth the time.