I think the legal no no's relate to mixing different ply's i.e radial and x-ply or say road and off road. Its one of those personal decisions you have to make with a bike - I tend to agree with Nicki about the possibility of an issue with insurance however small that might be. Why take the risk? If I had to mix tyres I would at least make sure the tread patterns were similar. Personally I always match pairs as each tyre has its own characteristics and I prefer to ensure these are the same front and back. 99.9% of the time no one would notice the difference but do you want to find this out when its wet and someone's pulled out in front of you? My view is that the 3 inches of black stuff we ride around on is the most important and critical component on a bike (just ahead of brakes) and don't mess with it. This is my opinion - you'll get plenty of people tell you the opposite and that they've ridden for 50K miles on an old tractor tyre at 12 psi and never had a problem.
