Its up to the rider, some people say they cant ride a bike with a new tyre on one wheel and an old one on the other. Most people can cope ok, you just have ot understand that with a near rear and part worn front that after the rears beded in the fronts going to be the weaker of the tyres ie least grip or other way round for new front and old rear.

Personally i dont really notice a massive difference in grip as long as the tyres are completey worn or squared off.

__________________
Plough on