I wouldn't say anything is wrong so long as you use similar to stock size. I would only lower it if you are changing all 4. If you have ever noticed, most BMW lowering springs lower the back less than the front. So if you were to just put lower springs on the back, it would make the back a little lower than the front stock setup. It probably won't cause any problems, it just might look a little weird. BMWs stock already have a lower look to their stance. You should probably remain uniform in what you do.
I agree with Furious, take advantage of this to do some upgrades with performance parts in mind!
5 Speed Manual
Alpine CDA-7895 Head Unit
12" Alpine Type-R, sealed box
Pioneer 400w amp for $20 at a yard sale (to replace my fried MRV-1507)
Infinity Kappa 4" 2-way speakers(rear)
Infinity Kappa 5.25" component speakers(front)
Debaffled airbox, K&N filter
Tracked Mud on carpets
Shampooed and cleaned carpets