What are you using at the moment? I'd put on Roadtecs if you want long life, Pilot Powers if you don't, but I'm biased
In theory, the two things you want to avoid is having a stickier tyre on the back than on the front- because if you're going to loose traction you want it to happen at the back front, not the first- and having drastically different tyre profiles. That's the idea anyway.
Personally it doesn't bother me too much either way- I've been riding with an Azaro on the back and a Roadtec on the front for months and it was just fine. Felt a wee bit off at the back at first, since the Roadtecs are round and the Azaro's are flatter, but it was never a problem.