Yeah, the "Ale Pail" type fermenter or a 6.5g glass carboy should have plenty of head-space for a 5 gallon recipe in 95% of cases. There are times (like pitching directly onto a yeastcake, etc.) that you will get blowoff out the airlock or it will clog the airlock and blow the entire lid off (can you say mess?) or blow out the airlock (hopefully you don't clog an airlock on a carboy and explode the glass carboy, ouch!)...
I wouldn't say this is 'rare' b/c I've seen several people describe this occurrence on these very boards (including an exploding glass carboy), however 19 of 20 5 gallon recipes should ferment fine in the Ale Pail or 6.5g glass fermenter.
You definitely should use a blow off tube as an airlock on any smaller fermenter (5.5 gal capacity with a 5 gal recipe for example)... If you do use a blow-off tube as an airlock, I'd suggest getting 3/4" - 1" tube as the smaller ones will clog and could cause the same problems noted above if the tube doesnt blow out of the hole...