I would recommend using canned/compressed air when cleaning things inside a PC. A soft brush isn't necessarily bad, but you should avoid contact with these components when at all possible. If the option prevents itself, get an ESD free vaccuum to suck up the dust
This so funny, i was at my friends place today and i was i saw a soft paintbrush in his living room, and he was like hey thats my painbrush that i use to clean the cpu... i was thinking it was werid, but i guess others do this too..
and.... if you're trying to spray canned air into a tight corner..... do not hold it upside down and try to spray it. that is..... unless you want a slightly frosty motherboard or whatever you're trying to blow off.
A soft paintbrush is not the best due to the possibility of creating a static charge fron the friction of the bristles on the board or any of the other things you rub with it. Think of all the rugs you use to rub yer feet on and then touch yer friends in the spine/back. Yeeeeow! Static electricity, bad ju ju.