Fork spring pre-load

I have an '01 426 with stock springs front and rear. The fork springs feel just a touch to light for me. Am right on the edge as far as weight goes between using the stock springs and going up one rate. I was thinking of trying maybe a 5 mm spacer to give it just a little pre-load and see if that would be enough to get it to my liking. Is there anything technically wrong with doing such a thing? If not, should the spacer go on top or under the spring, I have heard it makes a difference between suspension mfg.

Thanks,
Jason