I just tried the long tail cast on for the first time, after realizing I had been doing a knit-cast on this whole time, and now I’m wondering, what do I do with the tail? Do I just knot it off?
After you’ve knitted a few rows, go back and weave the tail in.
it is always best to weave in the tail while finishing. Knotting is never recommended.
It depends on what you’re knitting and the yarn you’re using. If you are going to be sewing a seam at that edge, then leave the long tail and use that as your seaming thread-- unless the yarn is bulky. You don’t want a bulky seaming thread. What you do is that case if find matching yarn which is much thinner, or if the yarn has a proper ply, split it.
The rule of thumb is not to knot, and wool has little fibers in it which grab onto each other (which is why it felts). But there are some very slippery micro-spun and bamboo yarns, which, if not knotted, will come undone with some wear and/or washing. So weave the end to the inside and discreetly as possible, knot it once, weave some more, knot once again. This keeps you from having a big, bulky knot, but it will be very secure.
I’m going to be another who recommends the weaving in. Unless you’re doing something like a blanket or scarf that has tassles, it’s never a good idea to knot as it is very unsightly once the item has been washed.