What size tyres are you running on 18inch rims?

I have got 215/35/18 on front and 225/35/18 on back.....now apparently this is not good for the centre VC over time...is this true its only like 1.15% different radious? They look like pram wheels from the rear as it is let alone going back to 215! Plus apparently the 225 wont fit on front?