ok my apologies, i was speaking of one thing and seems you were asking about another.
by touching metal once before starting, you remove any static and then go about the job. you both are speaking of staying grounded the entire time you're working inside the box.
either way is fine really. both do the job. i have ben building/repairing pc's for well over 20 years and all i have ever done is what i described above. i have NEVER had any issue with static and have never bothered with wrist straps and the other constant ground methods folks say are 100% necessary.
but if it make you feel better then leave it plugged into the wall for safety sake. again won't hurt anything.
but do make sure if you step away from the system and then come back, touch metal before starting again to remove any static build-up that may have occurred while away.
but in either case, enjoy the cleaning. i find it somehow soothing to sit there with some music on and a pile of qtips and other materials and just clean every nook and cranny