If you're at all concerned about needing a blow-off tube in the carboy, I would install it right after pitching the yeast in... Once it's done with spewing forth foam and other things (usually withing a week of fermentation kicking into high gear) you can remove it and install an airlock in it's place.. If the carboy is PET/Better Bottle, then you can use the same tubing as your racking cane uses in the bung's hole... If a glass carboy, you can do the same thing, OR get a length of tubing that goes directly into the carboy's bung hole and then into the bucket of sanitizer solution. If you go the glass route, just be sure you push the tubing through the bung, so that it it won't get pushed out. It should be a snug fit (as with most things going into bung holes
)... If you're using a PET carboy, then just push it in enough to go through the bung's airlock hole...
Another option available (some people use it) is to add a drop of fermcap per gallon in the fermenter...
So, you have a few options available to you... I've yet to have an airlock go airborn, but I've also used blow-off tubes in most of fermenters (at least initially)... I used my 6 gallon carboy and had foam through the airlock (~5 gallons of wort went in) about 6 hours after pitching the yeast (the first time I used a starter in fact)... Used a blow-off tube on that one and it was ACTIVE for the first several days...
Knowing how to set up the blow-off tube is an important piece of knowlege IMO... It means you can ferment in a wider range of vessels... Including corny and Sanke kegs. :rockin:
BTW, I think it's better to install the blow-off tube on the fermenter for the first week and NOT need it, then not install it and come home to a wort mess where you're fermenting...