My favorite version of the "should I or shouldn't I?" re: using a blowoff tube was recently when we tried a yeast immobilization batch. In theory, this would ferment very slowly and take a long time, with minimal krausen, so you SHOULDN'T need one...but I figured, ehh, let's rig one up just in case...you never know what might happen.
Well....heh...we must have been a tad bit off in one of our solutions, as on day 4, the yeast started escaping their semi-permeable homes. So some measure of 4 packs of rehydrated S-05 got loose in like a 1.080 beer mashed low for fermentability....