if theyre high quality(thick glass) they wont crack, if its cheap quality due to the extreme change in temperature it can crack
Boiling probably won't make your glass crack right then, but it will definitely weaken your piece and make it a lot more likely to break in the future. There are a lot of more effective/less risky methods to clean a piece, i would definately reccomend against boiling, it will decrease the life of your glass.
It's not so bad to boil it (although doing it often will eventually cause it to crack or break under pressure) but when you boil it, do not I repeat do not try to cool it down quickly by running it under cold water! That will make your pipe crack because of the extreme temperature, unless it's pyrex which is designed to withstand temp. changes and whatnot. And like sensiblazer said, there are much better ways to clean your piece.
I dont know about U guys, But I FuckN HATE Boiling my Glass. FUCK DAT SHIT! Ive Always Used the Rubbing Alcohol & Salt Trick! Its Fast, Easy, and it literally makes it Look & Smell Brand New. [ame="http://www.youtube.com/watch?v=BmRsT_sn-Ew"]YouTube - An easy way to clean your bong[/ame] U Dont Even need Sea Salt to do it, Just Rubbing Alcohol & Regular Salt will Do. Just Pour in the Alcohol, then the Salt, Shake it like Shake and Bake, then Rinse it CLEAN! Works for all Glass, even ur Glass pipes.FUCKN EASY!
I use to boil my pipes until one day i did and my bong slide practically shattered in the water.....piece was thick and shouldn't have busted but all pipes can have weak points. DON'T DO!!! use hot water and or rubbing alcohol and salt while shaking.....