I have paddled with some excellent whitewater kayakers who always roll on the same side. Obviously, being able to roll up on both sides is an advantage in some situations.
As Celia and others said, current, hydraulics, waves, or obstacles might make rolling up on one side much easier than the other. In a few instances, it may be impossible to set up or roll up on one side.
The very best whitewater boaters instinctively seem to know which side to go to and often don't even need to set up or go through the motions of a complete roll to come up. That ability might be a good thing to emulate, but I think most people take a long time to get to that point, if they ever do.
Among those boaters I know who can roll up on both sides with some facility, some will always go to what they feel is their strong side after a failed brace. Others will simply drop their paddle down to set up on the side of the failed brace, and utilize the momentum of the capsize to get their upper body toward the surface as the boat rotates then roll up with their paddle sweeping on the opposite side of the boat so that the kayak rotates 360 degrees.
If you don't want to do that, I agree with jcbikeski that it is often necessary to hesitate and let the boat settle flat before starting the sweep, otherwise the momentum of the capsize might prevent you from getting your paddle up to the surface.
What I would suggest is following jaybabina's advice and keep it simple at first. Decide which side is your best side and go to that side first. I would suggest getting in the habit of going to the other side if your first roll attempt fails, however. It is usually much easier to set up on the side of a failed roll than to horse the paddle through the water to the other side to set up. That way, if a wave, current, etc. is making it difficult or impossible to roll up one on side, your second attempt will be on the optimal side.
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