Unless iPhone is syncing with your computer, you can disconnect if from your computer at any time.
When iPhone is syncing with your computer, iPhone shows “Sync in progress.” If you disconnect iPhone before it finishes syncing, some data may not get transferred.
You may want to prevent iPhone from syncing automatically when you connect iPhone to a computer other than the one you usually sync with.
Turn off automatic syncing for iPhone
Connect iPhone to your computer, then select iPhone in the iTunes source list (below Devices, on the left) and click the Summary tab. Deselect “Automatically sync when this iPhone is connected.” You can still sync manually by clicking the Sync button.
You use the iPhone settings panes in iTunes to specify the iTunes content and other information you want to sync to iPhone.
Set up syncing
When you connect iPhone to your computer, iTunes syncs iPhone with the information and media on your computer, according to how you’ve configured the iPhone sync settings in iTunes.
You can set iTunes to sync any or all of the following :