Can one be electrocuted when cleaning a light switch?
Wall switches are designed to be reasonably safe, at least under sensible conditions of (ab)use (with the cover plate on, of course). 

In general, don't spray cleaner or similar liquid of any kind into a switch-- rather you should spray the cleaner onto a rag and wipe the switch operator and cover plate with the damp (not dripping) rag. Ordinary disinfectant should be fine if the label says you can use it on plastic, but solvents are a no-no as they could attack the plastic and embrittle or dissolve it. 

There's no reason to turn off power on any properly wired residential switch I've ever seen for everyday cleaning (at least in the first world, I've seem some pretty dodgy stuff in the 3rd world including open knife switches with 240VAC/50Hz exposed).