I am wondering: Is it really necessary to prick potatoes and sweet potatoes before baking them? In what context? Or is this just a cooking myth from days of yore?
No, it's not a myth. If a potato happens to have a lot of internal moisture and a skin that is a bit thick, the build up of steam can and will cause it to blow out in the oven. Or the microwave, for that matter. My children have all created messes in both oven and microwave, cooking potatoes in the skins without pricking them. You don't have to go all crazy with the fork, but you really should poke your tuber at least one time with one. Be sure to get into the flesh, not a shallow poke just in the skin. My mother always told me to poke potatoes, and I thought - as a wise young person- that it was hokey old wives stuff. Didn't poke for a long time, and then I had a rather large russet potato absolutely explode in the oven. It made a stinky mess to clean off the oven walls and racks. Since then I poke, and I've had no further incidents. Well, if you don't count my kids. They used to think the same thing of me that I thought of my mother. Now they poke too.
Absolutely, I forget every once in a while and they usually blow. You haven't experience mess until you blow up a nearly done sweet potato. Sausages too.
I saw a FN cook say to use a small skewer and make a poke at both ends to let steam release. It works.
Totally agree! ATK discussed this on one of their shows and although their cooks could not replicate an exploding potato, they said for the time involved, why wouldn't you give a couple of pokes to ensure all the taters remain whole.