too low can cause errors in memory, too high and it slows performance
quite a flexible timing in a way and can be lowered for benching - (super pi benches usually benefit from a low tRC)
the minimum values for tRC should equal tRAS+tRP
i have my mem set to DDR1000, 4-4-4-10 at the mo so that should give me a minimum of 14 for tRC (i've set it to 15 - memtested for over 94hrs so far)