A little bit of help lads?

I've had another look at the rears; there's oodles of clearance between the wheel and strut (perhaps 20mm or 3/4") , but very little between the tyre and strut (about 5mm or 7/32"). Is it just the tyre size in relation to offset that matters? So by running a tyre very close to stock width, on a wheel with offset close to stock it should fit okay, even though the wheel is an inch wider?
Just about every one here has after market wheels, so what wheel size and offset do you all run?