I've always used the back of a knife. It doesn't make a huge difference but throwing the sparks with the back of a knife blade is a little easier if you square an inch or so of the back with a stone and leave the edge semi-sharp.

I have used the normal cutting edge of a knife but it tends to dull it and can create notches. It isn't a big deal with a cheap, semi-disposable, knife but messing up the fine working edge on your good knife is just wasteful.