One of the big questions I see from A LOT of microstock photographers is whether it is worth going exclusive with just one company, or better to take multiple agencies to sell images on. It is a difficult argument really, due to many factors. Namely, being exclusive with an agency usually grants a bonus percentage per image sold. Usually 20%-40% additional per image. It is definitely a significant amount of an increase, considering total revenue.
If you are planning to upload to 2 or maybe 3 agencies, going exclusive with one of the bigger agencies may be preferable (and by big, you can check here to have an idea of what I mean). However, in general, pushing for multiple agencies, if you can stomach all the uploading hassle, is worth it. Remember, not two agencies or maybe even three will be worth it for multiple agencies, we're talking 4+ easily if you are not going to go exclusive.
Don't take my word for it. Yuri Arcurs (arguably one of the most successful microstock producers) agrees. Check out his article, especially the bottom portion where it lists the best sites and why it is beneficial to sign up for multiple agencies, rather then one. After you finish reading up, come back and upload to Fotolia, Shutterstock, and a few of the others... and get down to business!