It's probably fine. Just don't try to use the new metadata designer. That specific feature requires Laserfiche Repository Server 12. Repository Web Client 12 may handle trying to use the new metadata designer with Laserfiche Server 11 gracefully, or it may not.
Edit: Tested it out and it's handled gracefully. Nothing crashes or fails in a detrimental way. That was our main area of concern, so unless you run into some other issue, I don't see any reason to downgrade back to 11.
Edit 2: We don't recommend anyone else do this. You're not supposed to be able to connect higher version client applications to lower version server applications. There's normally an explicit check for this, which is either missing in Repository Web Client 12 or wasn't implemented correctly. We'll be fixing that in the next release. Repository Web Client 12 is working with Laserfiche Server 11 coincidentally, not deliberately.