If you're not going to use the OEM upper spring perch (and we do not as you can see in strat61caster's picture of our RCE coilovers), then I would highly recommend using the OEM conical washer.
Details in our instructions here:
http://racecompengineering.com/instr...structions.pdf
Our RCE coilovers are made by KW so instructions are similar for your STs.
- Andrew