In general, I think it's a good idea to have at least a few mm of spacers so that you can potentially switch to a different headset at some point in the future if you need to. That way, you're not locked into looking for the same stack height or shorter when you're shopping for that next headset.

So a spacer is my vote. It's slightly more elegant to have a single spacer that's the right height, but no one should get on your case if you have two or three spacers in there.