I had two thoughts. First, tire warranties are becoming less and less useful. If your tire wears in less than the warranted mileage, you'll be lucky if they pro-rate the difference on a new set of tires. They can even get around this by citing misalignment, improper/insufficient rotation/balancing, or even improper pressure. It's extremely hard to argue against any of these because all of these things will lead to accelerated wear. If any of these are cited, you won't get anything back for your warranty, since it only covers "manufacturer defects" rather than normal wear. I work in elastomer (rubber) research and the last time I had someone look at my tires, I told them I was pretty sure the tires wearing out in less than 50% of the warranty is a defect resulting from the manufacture. They didn't think much of that and basically I had to eat the cost of the tires. They cited either misalignment or insufficient rotation, which I had done myself, so there wasn't much I could do.
The second thought is that if you're going to pay someone to rotate the tires, it's going to come out to more than you'll save under the best of circumstances. If I have a set of tires rotated 3-4 times and it's only $20 each time (very, very low estimate

), then I'm still coming out behind because I'll never recoup that expense even if they pro-rate the warranty, which is not a given.
So, my recommendation is to just do it yourself. You're not likely going to get much love from the tire-man, but he's not really interested in helping you out anyway, so this is almost always a better financial decision.