I have done it many times and always been careful to not touch any of the components.
I did it today to clean my dust filter, I would like to be a bit more adventurous and clean my GPU fans and heatskin while they're inside my PC using my vacuum - covering the nozzle with rubber to avoid static shocking my components. Also is there any harm if my GPU fan starts spinning while my computer is switched off?
I would say its fine as long as you keep it well away from components, so fine for the base where lots of dust collects.
But you get into nooks and crannies you can use compressed Air cans.. but im cheap.. and have always used a drinking straw,, and blow..
Works very well for CPU coolers etc.. (try not to spit!!)...
If you cant do it outside (it throws a lot of dust up in air) wait for dust settle and then vacuum from the base.