On a hardtail. The front's got less weight on it, a suspension fork and a bit more care about where it's placed compared to the back. It seems like a pretty sensible idea to have different strength wheels front and back. I think I remember reading Sheldon Brown saying that if the front and rear are the same, either the front will be overbuilt, or the back will be underbuilt.
No-one seems to talk about doing this though. You certainly don't see it on complete bikes for sale with different wheel builds.
Why?
Are people worried about different tyre profiles ruining the handling? Keen on savings from a buying a pair?
Or just deeply suspicious of things that don't match?