If one foot's cleat wears quicker than the other due to riding style, is it a good idea to switch them after a certain time to extend the lifetime of the pair?
I see from your other question that they're Look Keos.
As they're plastic, they're going to wear quicker than a metal cleat, and yes, you could swap them around.
However, I think a better strategy might well be to replace them one at a time as they wear out.
Plastic stuff doesn't really like being repeatedly removed and reattached in most situations and you need to keep them in the correct positions.