There are obviously a lot of opinions on this and arguments can be made either way - what I do notice is that most fast setups that I know about seem to have spring rates front and rear within ~1 kg/mm of each other - front biased, rear biased, or even.
The fastest STX twin (autocross) was running KW V3's with the front and rear springs swapped at for 400/340 F/R, in addition to a stiffer front bar.
RCE and CSG seem to prefer even rates or more rear-biased, depending on stiffness it seems (what Andy was talking about keeping the frequency bias the same vs the spring rate bias).
Here's some numbers I put together for spring rates, wheel rates, bar rates, and how it all contributes to frequency:
http://www.ft86club.com/forums/showp...&postcount=107
Yes, it's just one piece of the puzzle as there are so many other factors (ride height, travel, alignment, DAMPING), but the numbers are still useful. This should at least help if you are looking to keep the NF "balance" the same as you go stiffer overall.
And also it will really depend heavily on driving style and talent.