Depends on the style of the comic!
I use them only very sparingly - partly because the kind of comic I draw rarely requires much use of them, and also because I haven't got very much practise drawing them, so I limit my use. I imagine that if I was doing a more humour-based comic, I'd use them more often.
My rule of thumb is to only use them to emphasise what I'm doing; if readers can't tell what's happening without the sound-effect, my drawing isn't clear enough.
As for "BOOM" and other common sound-effects - that's perfectly fine! If you want people to know something explodes, using "BOOM" means they'll get it immediately, because it's a sound-effect they're used to seeing for explosions! Don't stress out about being 100% original every time - comics are a language, in a way, and the thing that makes a language useful is that everyone has a common undertanding of some things. Innovation is nice and necessary, but you don't have to do new things all the time. It's okay to use common sound effects, because they're easily understood.