It really depends on what you want to get out of it. If you put a shim in there, it will raise the rear ride height and the bike will turn in more quickly, sacrificing some stability. When you start playing with chassis geometry, you should really know what you're doing otherwise you're going to create some problems. I had an 02 gsxr 600 for a while, put a 10mm shim in there (under where the shock mounts to the frame) and it felt like a little too much (for my taste). 6-8mm would probably have been perfect. I'd also recommend re-doing the forks (stiffer springs at least) if you're going to raise the rear ride height as you will be putting more weight on the front of the bike by doing this.
It's pretty easy to do, just need a buddy to help you. Remove seat / tail fairing / battery & battery tray. Undo the bolt on top of the shock, have a buddy hold up the subframe for you (raising it over the end of the bolt). Put the washers or shim in, put the subframe back on top of that, put the bolt on, and you're done. Play around with it - start with 6mm or so, see how that feels, then go to 10mm, see how that feels, etc.
Hope that helps.
