It isn't a bad idea, depending on what you are using as said "crack pipe". Hell they even sell "crack pipe" type vapor pipes specifically for BHO and concentrate. If it's the ghetto light bulb vaporizer, I say no... regardless of the cleaning of chems and what not, no. That's disgusting. My opinion though, your body you can harm whatever.
But I must add, it isn't the best way to enjoy your BHO. It always tasted a little
off and never hits as well as you'd want it to. But if you go down this road of BHO crack piping
I suggest a butane torch to heat your glass. Don't be an actual crack head with a Bic lighter or candle. Use butane, it's cleaner. Wont make black marks all over your glass when you heat up that P-LO!
I recommend this for first time BHO set-ups. As all it is, is a new bowl piece with a stone in it. Just remember to use butane only torches with these stones or they clog.
http://www.healthstoneglass.com/
This guy makes these out of San Diego, CA. Just put the stone in the bowl piece, put oil or concentrate on top, and just apply some heat with a butane torch.
Happy toking.