Before concerning yourself with spacing, be sure the spacers you have are hub-centric. I will explain.

Many cheaper spacers are not machined to fit tightly over the nose of the hub, which is what should support the forces wheels have to cope with. If you take your wheel off and offer a spacer up to the mounting face, a quality spacer will slip onto the nose of the hub just like your wheel does. If offering it up you discover that it slops around or for whatever reason does not fit the hub nose well, mounting wheels with such a spacer will be asking the wheel lugs to cope with both clamping load and shear load, which is unsafe.

Keep in mind also the length of your lugs. Are they long enough to reach through both the wheel AND the new spacer?

These are things to keep in mind when using spacers to either correct for wheel offset or to widen track with stock wheels.

This link explains hub-centric better than I can.

http://tires.about.com/od/understanding_wheels/a/hub-centric-vs-lug-centric-wheels.htm