Shutterstock: In order to contribute at Shutterstock you first have to submit a sample of your work for consideration. Of the ten images you submit, seven or more must be deemed acceptable in order to pass. If you do not pass the test then you must wait 30 days before you can apply again. As Shutterstock are likely to be very important for your indie income it obviously makes sense to apply as soon as you have made the decision to leave iStock. This gives you time to reapply if you don't pass at first without losing potential income. The only problem with this is that (unlike at iStock), any images that pass the test are automatically accepted for sale and go live on the site very quickly after. Clearly this would violate the terms of your iStock exclusivity agreement if it happened before your notice period had expired.
The good news is that it is possible to make your application and be accepted without images being released for sale. This is done by 'opting out' of sales on Shutterstock. The way to do this is to first open an account at Shutterstock (my referral code is 661795 btw *wink, wink*), then go to your homepage (the one with the world map on it). At the top of this page you will see a tab marked 'Resources'. From this menu select 'Your Account', then scroll down until you see a list of sites with 'opt in, opt out' radio buttons beside them. Select 'opt out' on them all and your images will now remain in the dark until such time as you go back and select 'opt in'.
So, assuming you pass the contributor test and have followed the above steps to hide your images from the buyers, there is no reason not to upload the bulk of your images to have them ready to go live on the site at the click of a button once your exclusivity falls. Or is there? Well, people who have done this have reported very poor initial sales on their images. It would seem logical that the Best Match position for an image be determined by its sales since acceptance, as it is unlikely the system will be aware of whether you have opted in or not and when that happened. With huge numbers of images being accepted on Shutterstock daily, it is very important to gather sales in the vital but short-lived window when new images are at the front of searches. This ensures they will remain there - images that do not sell during this window may struggle in future.
Dreamstime: Dreamstime don't have a contributor test so there is no urgent need to open an account early (referral code for when you do though! res2635685). While they will temporarily suspend your account if you email them and ask them to do so while you wait out your exclusivity notice period, there are no benefits at all from doing this. Any images you upload while your account is suspended will not be inspected and will only join the queue for inspection once you get the suspension reversed. *edit* I have since heard that this may have changed since I opened my account there in October 2010. It may be best to check with Dreamstime directly to see what the position is now if this is something you think is really important to you.
So in short it would seem a good idea to apply in advance to Shutterstock (unless you are very confident of passing the test first time) but not to upload any further images ahead of time. Of course this means that your Top Ten images used in your application would suffer from poor Best Match placing once they did finally go live for sale, something you would have to weigh up in your decision. It would seem best to bide your time and wait at Dreamstime too. Their queue tends to run at around 4-7 days, so starting to upload a couple of days before D-Day should be safe enough. Best of luck to any iStock exclusives that decide to try independence over the next few weeks. Whilst I would be pathetically grateful to anyone who decides to use my referral links there will be no hard feelings if you don't. It would be good to hear comments from those of you who are considering making the change - these can be made anonymously if you prefer.