All the spacer does is keep the tube from extending all the way out. The only pressure that is on it is from how much preload you have. PVC is cheap and plentiful and easy to work with, so why not. But you can use steel, aluminum, brass, whatever, so long as the ID and ODs are right for the damper rod.
If you cut 3" off the spring, and add a 3" spacer, you will have the same preload as stock and be 3" lower. If you cut 3" off the spring, and add a 4" spacer, you will be 4" lower, but have additional preload over stock, i.e. a stiffer front end, which depending on how you ride or what you weigh, might be needed when you lower the bike. If you add a 3" spacer and don't cut the spring, you will be 3" lower and have a super stiff front end, if you can even get it back together.
Same goes for cutting top tubes. Cut 3", but keep the same length spring, and you will have a pretty damn stiff front end. Even cutting a .5" off the spring might make it too stiff, but it's easier to cut more than to add it back, so....
I'm real happy with the way mine rides, and when I switch the lowers out and add spacers, I'm not really planning on adding much, if any, more preload than I have now. The preload I have now is enough to raise the front of the bike when I get off of it. Spacer will keep that from happening, but should ride the same.