I wouldn't risk it personally, tyres cost less than centre differentials (actually that might not be strictly true but tyres are easier to have fitted). If the rolling radii of the different size tyres are close enough it might not be any worse than continually driving in a circle.
I was once advised by a tyre fitter not to have 235/70 back and 225/75 front but then he wanted to sell me more tyres - I didn't take the chance and coughed up for the extra tyres.