I have an E90 with 17" tyres, all the same size.

I decided to swap the front with the rear as the rear were wearing out faster - but I was sorry that I did.

After I swapped the tyres (a few thousand KM later) I had to change all 4 tyres as they were wearing really strange. It seemed that the font and rear wear on the inside and outside differently. I ended up with what looked like a ridge in the middle of the tyre (and it didn't look good).

When I was changing the tyres the BMW service guy wanted to start adjusting the rear tracking (I think he said tracking) because the tyres looked strange - then I told him what I had done and he said that was the cause.

I reckon I would have got more mileage out of my front tyres if I didn't do the swap