I believe common wisdom has been 17" wheels for winter driving with all four of the same size, not staggered. 17" wheels are slimmer and thus plow through snow more easily than 18". Staggered is a no-no for basically the same reason, bigger back tires means harder to plow through the snow.
And on an XI; unless things have changed I didn't think BMW recommended staggered set ups for an AWD models. At least in the E46's that was the case.