What I do, is I rename minecraft.jar's that have mods on it to minecraft-mods.jar and I use the vanilla one to play on opticraft. When you are not on opticraft, name that to minecraft-legit.jar and name the minecraft-mods.jar to the original name, minecraft.jar. To get an original minecraft.jar, rename your current one to minecraft-mods.jar and just force an update.
I never play single player anymore, so I don't need to do that. But when that new rule came in, I did that.