Ok, I made a quick little video of my XXX-CR. (1/10th 2WD buggy) Front springs are red (2.5 rate), rear springs are white (1.8 rate) Best I can tell the back has a much lower frequency rate and must be brought up to match the front since I have the softest springs available in the front for this car. What do you think? How far "mismatched" does this appear to be in your opinion?

http://www.youtube.com/watch?v=f20gDe6BeNc

Also, most drops were made from about 2-3" as anything above that would bottom out the rear chassis.