all that keeping the same offset will do is give you the same track width. how many track GTR's etc run 18x10 +12's and the likes without any issues.
and for those saying 7.5" wont fill the guards, blah blah.. you obviously have no idea about offset.
a 7.5 +15 for example, will sit further out than a 9 +38...
that said, i don't recommend 7.5" rims...
something around 18x9 +20 front, 18x10 +25 rear will be nice and tough on a 33/34.
i run 17x9 +15 all round on my 31, but the rear could ideally be 17x9 +0, or 17x10 +12 to fill the guards properly.