In my opinion, there is no downside of leaving it on. Unless your having some stability problem in an OC CPU.
Disabling EIST and C1E will stop it from changing. The downside is you loose the ability to reduce heat when its not being really used. You do get a small power savings as well, but that really doesn't matter much.
Even when you do OC, as long as you test it, speed step can help reduce heat when its not being used. Although in the past I thought it was best to disable it when OC since going from 1.8ghz to 3ghz was quite a shift. In my mind, I thought the system would freeze. I read that it wasn't the case, and even tested it myself.
With my system (E4400) OC to 3ghz and idle at 1.8ghz, my idle temps are mostly like stock speeds (so it actually should be 1.2ghz idle, 2ghz normal speed).