Well first take into account:
1.) Tire Shore (hardness of tire)
2.) Tire Wear
3.) Run Time
4.) Front to rear split ( depends with car and pulleys, check instruction manuals)
5.) Driving Style (how hard are you on throttle?)
Basically you want to cut down the tires to were the car handles at its absolute best, then you should check tire wear for every 5-8 minutes of run time (usally amount of qualifier) Then depending on what tire wear is.. cut to desired size for racing.
Hope this helped