You wont wreck the o-rings but you will take some life out of them. O-rings will dry out and boiling accelerates that. Best to take them off before you boil and put them back on after.I've put my plastic M-tanks into the boiling water, but not for the whole time -- usually just a minute or two, right at the end of the 15 minutes. (and always left the o-rings on... it didn't seem to hurt them a bit).
Andria