I ferment 5 gallon batches in 6.5 gallon carboys. Been doing so for 6 years. Probably a couple hundred batches.
Never used a "blow off", just use an airlock every time. In that time there has been only one time where the fermentation was so strong that the airlock couldn't handle it and the stopper got blown out of the carboy. And on that occasion I just squirted it down with star san, stuck it back in, and the beer came out fine.
Blow offs are a waste. Only the paranoid people use them. There's really no need. Airlocks do the job just fine. All you have to do is make sure to use a large enough container for your primary.
IMHO, blow offs are sort of a noob thing. If you hit your gravity right, and have the proper container for a primary, then an airlock is all you really need.
Blow offs are something you use on special occasions, when you're making an extra special, super-high-gravity brew.
You don't have to use them every time.