Only if you find having the handlebars lower is more comfortable to you. Otherwise the spacer should be moved under the stem. Then pre-set the top-cap by tightening the cap down about 1/8th turn with your hex-wrench and then tighten your stem - side to side - to the manufacturer's suggested torque. Use a torque-wrench. Then your stem should have no play, and your handlebars should turn freely to the side when you lift the bike off it's front wheel and tilt it side to side. Don't torque-down the top-cap. Just tight enough so it stays put.

Most of the job is done with the stem.

Here's Park Tool on the subject:

http://www.parktool.com/repair/readhowto.asp?id=65

And BicycleTutor, of course:

http://bicycletutor.com/adjust-threadless-headset/