few tyre questions ?

Hello boys and girls . Have to replace 2 rear tyres in the back of the 520 very shortly . My question is , what is the point of bmw approved tyres I.e. the star on the side wall ?
The car is on 20"s so I know tyres will be expensive but im not really prepared to fork out an extra wad of money jus to get a star on the sidewall !
I also have a set of 17" wheels for the car with 2 perfect Goodyear winter tyres on them but the other 2 have no tyres at the moment . So is it time to sacrifice looks and put on the winter wheels then worry about the other tyres next march or so ?
Does anyone know of any deals at the moment on 275-30-20"s or 225-55-17" winter tyres ?
Thanks lads

I wouldn't bother with BMW approved tyres, its a waste of time as it wont effect any warranty the car may have. Its just a gimmick