I think it depends if you are building a Sierra based or MX5 based Roadster.
The MX5 rear track is smaller than the Sierra rear track (1427mm VS 1468mm). As most of the body kits have been designed for Sierra based Roadster, rear wheel arches are too wide for the MX5 rear end with original wheels, and I often noticed on MX5 based Roadsters that there is an unpleasant looking gap between the outside edge of the rear tyres and the inside of the rear arches (about 20mm on each side). It looks like the car is too small for the bodywork.
On a Sierra based Roadsters, the original Ford Sierra wheel offset (ET35) is OK for both front and rear. Front and rear tracks are exactly the same, and the rear wheels fit nicely in the rear arches, close to the inside edge without never touching it.
But on an MX5 based Roadster, of you keep the original offset (ET40 iirc), you will have the weird gap I'm talking about. So if you can, chose smaller ET (bigger offset), to completely fill the rear arches, the car will look much better.
This problem can also be solved afterwards by using wheel spacers.
Hope that will help