Is there a reason not to use the same tire size on all four (rear) wheels? That way you could rotate them...
No reason not to run four equal sized tires if the only change is swapping the front rims for rear rims.
For me, I'm doing a couple of other suspension mods that may make retaining a tire size stagger a good idea. First, I'm currently running 20 mm spacers behind my stock front wheels. This made a significant difference in reducing understeer and reducing tracking/wind sensitivity issues. The front wheels are 4" closer to each other than the rears, and the 20 mm spacers reduce this amount by almost 40%. The increase in track width dynamically softens the front suspension and creates a better turning arc. What I mean by that is that with the front wheels pinched together, it becomes more like a tricycle - you get this weird lean because of location of the outside front tire.
I'm also planning on stiffening the roll resistance of the rear suspension by changing the center bushing for a harder compound. There's also an outside possibility I may install a rear anti-roll bar. Both of these changes will reduce grip balance at the rear wheels.
I found out last winter that despite a whole lot time spent on a race track, that I'm not good enough of a driver to reliably catch the smart if it starts to spin, and that was in a slow speed, low grip condition of being on snow and ice. Once it starts to pirouette, it required an inordinate amount of correction, way more than I've been used to, and combined with the slow steering made it very difficult for me.
With all that, I feel like it'd be better to retain a small stagger, and live with a small lack of front grip rather than having something a bit tail happy. I feel relatively confident I can get the handling and grip balance with small changes in tire pressures. For everyone else, just going four equal sized tires is probably the best bet.
