All depends on what you can set them too, it's up to you and the capabilities of the DRAM, If you have good sticks, say a pair of 2133/9 and a pair of 2400/10, you quite possibly can run all at 2400 (I've done this many times, you often need a change or two to the DRAM and MC voltage, also may need to go into the Advanced/secondary timings - as mentioned with good sticks can often OC the lower rated ones, or just drop the higher freq down to the lower - can do some interesting things, I've mixed 1600 and 2133 sets at 1866 even