I wouldn't shim the cartridge with washers. First, I would consider what you might gain by trying to adjust VTA. You don't mention just how your set up sounds right now. So before you go achangin' I would do some thinkin'. What's going to change in sound quality in your system by jacking with the headshell?

If you want to experiment, try a thin piece of cork cut to the dimensions of the cartridge body. Otherwise, if the cartridge body isn't rubbing on warps and you have no complaints about the sound - only a comparison to what another system sounds like, I'd say leave it alone.

Maybe a thicker platter mat would help?

Here is a product that might give you the VTA you are looking for plus other benefits.

http://www.thecartridgeman.com/isolator.htm