For iOS devices it doesn't matter. These use a completely different, non-disc-based synchronization protocol, and of course with Wi-FI sync there's now little to no difference between a USB connection or simply syncing over Wi-Fi. Consider what happens when your iPhone rings while you're in the middle of a sync
Essentially, iTunes and iOS are communicating as two intelligent devices and iOS is capable of dealing even with an aborted sync operation without any problems, as it can clean up any open database files and partially-written media files or apps by itself.
However, traditional iPod models -- the iPod nano, iPod classic and iPod shuffle -- still mount as if they're external flash drives and sync via iTunes by direct file system access. This even includes the sixth-generation touchscreen nano, despite it's iOS-esque user interface. These devices should always be ejected prior to disconnecting in the same way as any other flash drive, especially
if you've just finished syncing. These devices don't actually participate
in the sync process -- they're "dumb" devices in that sense with iTunes doing all of the work and processing of writing files to the external device and opening and updating the database directly. Yanking them out during a sync will basically leave half-written files and inconsistent databases.