No it's not. You don't have to have the sync box checked to drop music onto your iPhone. Actually it's the opposite. If it is checked you can only add or delete tracks if they are conditionally set to according to the sync settings. Ie certain playlists etc. If it is not checked you can just drop music in, generally.
Devian, we need more info. Has it worked in the past? What method are you using to add music? Is the iPhone synced to another computer? Error messages?
Thanks for the help guys but I just decided to erase and add. I had all my music backed up externally before I got the new(er) laptop so its all good. Thanks though.