Well, the simple answer is yes.
If you run the RAM at less than it's rated speed you are running it sub-optimally.
But the real answer might be no. If you have enough headroom to overclock the CPU on a positive divider then do it. If your RAM won't clock hard enough on 3:2 or 5:4 then your overall system will still be faster with the CPU overclocked and the RAM running slower.
But as you haven't given any information beyond the absolute basics and you're asking what is, frankly, a pretty noob-ish question, then you can expect a low-level response. If you want to tell me what motherboard, the exact RAM specification, the stepping of the CPU and what level of cooling you have available then we can discuss your overclock on a slightly more advanced level.
I'm only trying to help. Try being patient/friendly - it gets you further.