I've had recommendations from tyre fitters that mixing is alright so long as the rear has less grip than the front. The idea being that if one of the tyres lets go it'll be the rear that slides and it might be a little scary but it's relatively easy to save it, whereas if the front lets go first you're stuffed.

So don't go running a Pirelli diablo on the rear and leaving the original OE Dunlop on the front :wink:

Tyres:
 

I have run different makes on my curvy with no problems. Currently running an Avon 45 front and Bridgestone BT020 rear. Has earlier post always run a stickier front if mixing compounds, easier to control a rear slide. Sports bikes are more sensitive to tyre choice due to steering geometry and power outputs.

Thing is, having a sticky tyre on the back doesn't make the front any more likely to slip, unless your approach is "Ride until the back slips then turn it down".