No you don't have to sync everything, with iOS devices and iTunes you've got a few options of what you want to sync.
The usual option that most people pick is to automatically sync the iPad with iTunes, that way if something is added to iTunes it goes onto the iPad and if something is taken out of iTunes it goes off of the iPad. Very simple, but somewhat limited.
If you connect the iPad to your computer, got into iTunes, click on your iPad under devices and go to the "Music" section you get a bit more choice....
You can turn off "Sync Music", that puts the music completely under your control. If you want some new music on the iPad you drag it onto the iPad from your library, if you want something off of it you have to go and delete it from the "Music" bit under your iPad. This is very useful, you have total control of what goes on and you can also delete things from iTunes without them being deleted from the iPad, very useful if you're pushed for space on your computer.
If you do have "Sync Music" selected you can the choose to sync just selected playlists, artists or genres. So then you could pick just to have music from a couple of artists on there. Or, for a bit more control, set up a playlist called "Music I want on on iPad" (or something) in iTunes and then tell your iPad to sync only from that playlist. You want new music on your iPad add it to the playlist, you want it taken off then delete it from the playlist.