i tend to swap them round after 5k miles to get front and back to be the same tread depth (and swap all 4 in one go)
but yes i agree with hurdy that new tyres should really go on the back for the understeer is safer reason.
but i tend to do the opposite...as i like the rear end to be a bit lively!
here's a question tho regarding space savers.
why do people put these on the front wheels if a car is FWD? are you not supposed to put them on the rear if car is FWD? I mean, the front wheels are doing the driving and steering so u'd want a proper wheel on there surely? I've used the space saver couple of times and i've always moved one of the rears to the front if it was a front tyre that got punctured.
can't remember if the manual says this or not tho!