Is it safe to use a device or disconnect it while syncing in iOS 5?
If you mean 'can I unplug my cable', then sure. It'll kill the process for that specific sync and you'll pick it back up next time. If you want to use your phone whilst syncing then you certainly can.

If you've gone wifi sync it'll just continue until either the network connection doesn't work, or the machine is stopped, or the sync finishes.