I agree with others above, check for uneven wear. If it's there, either live with the pull or replace the tires and have it aligned again with the new tires.

I'm a stickler on alignment, if it's off even a little it ruins tires fast. Even new cars should be checked, usually the dealer will do it for a small fee ($20-30) and if it's off, they get paid under warranty and it's free to you. The cars travel strapped down with the suspension compressed, and this can cause the factory setting to creep a little.

I'm a big believer in good tires, they're the only part of your car keeping you on the road. I drive about 45,000 miles/year so I have some time to think about this!

-Zeke