I presume that by "rally" you are referring to tarmac rallying as off road rallying requires soft spring rates.
I have used 180lb coilovers on the front and 150lb coilovers on the rear of my Mk1 Escort. My experience has shown that this works very well when used with appropriate adjustable dampers such as adjustable Koni's (which I am using) MCA, Bilsteins, GAZ, DMS, etc etc. Whilst spring rates are very important, the same amount of consideration should be placed upon the dampers. Having the ability to control the rebound and compression of the damper will only enhance the springs performance.
Cheers
Craig