I always use a blow off tube. I'm greedy and fill those fermenters as full as possible. Plus I hate cleaning yeast explosions. Since I started using o2 to aerate, it's become a necessity.
I just started using blow-off tube yesterday. Brewed a very BIG imperial stout. I modified an airlock and removed the water from it. Then ran some tubing from the airlock into a half-full growler. The tubing is secured into the growler with the growler lid having holes in it for the CO2 to escape.
I may need one on my California common. I didn't boil off enough fluid (my OG was about 8-10 points low) and therefore have a max of 1 gallon of headspace... I'll keep a close eye on the primary today and if it gets semi aggressive I'll switch to a blowoff tube.
It was always necessary when using a carboy, especially one too small. I've been using buckets and the blowoff tube seems quite unnecessary these days.