I put 6.25 gallons (maybe a touch more on occasion) in a 7 gallon fermenter. I always use FermCap. I always use a blow-off tube. Maybe 1 in 10 batches have something going up the blow-off tube, rarely do I actually lose any liquid. It is usually Belgian yeasts that causes activity in the blow-off tube.
I had a couple of problems early on with airlocks blown across the room and stains on the ceiling, that I use a blow-off tube every time, and would recommend everyone do the same. For the little extra effort it is to use one (when you are in the habit of doing it every time, it is very little extra effort), it is worth it to save the clean-up effort of 1 blow-off event.