I usually aim for a stability of 1.0, a stability of 1 is the Center of Gravity (CG) is ONE caliber (body tube diameter) forward of the Center of Pressure (CP). Anything less than one is considered to be marginally stable, and anything over 1.0 is considered to be over stable (iirc). Overstable rockets usually want to weather cock (turn into the wind) to varying degrees, marginally stable rockets may do everything but fly straight. The rules can change a bit for Short and Fat rockets, and also for extremely long rockets. Generally I try to get as close to 1.0 as I can, that seems to be a good safe number and is widely accepted. Someone else will be along later i'm sure to expound in greater depth on this fundamental design principle of rocketry.
The rule of thumb I use is between 1 and 2. Anything below 1 is unstable and very much a no-no. Anything above 2 is overstable, which is fine if you don't mind the weathercocking. I've heard that for Mach+ flights, your stability margin should be at least 2 (because at Mach 1, that 2 turns into a 1 because of the Mach effects on CP).