As the other two above have said, you certainly can.
It is very dependent on your kernel (please don't post your kernel and ask if you can OC with it now) and how your rom feels like acting. setCPU can cause instabilities and cause things to FC or even random reboots. As far as overclocking, I don't see any discernable reason to do it; I'm pretty sure the benefits returned are slim to none, and you're much better off from a stability standpoint to just lower your low-end frequency to 128 (many kernels will have screen off profiles that rely on 128) and be good to go.
Most decent and reputable kernels will already have a built in screen-off profile, meaning creating one in setCPU is completely redundant and unnecessary.
Personally, I use OS Monitor (from the mkt.) to lower my frequency and carry on with my day.