Good question MTSk&p, here's some wonders about it I've had too.
I've heard bb's are put in the tire when rolling inside prep, to keep the prep off the sidewalls.
Is that true? Seems like there's a multitude of ways to apply inside prep. From off the rim to on the rim, to bb's used or not, to heat applied or not, to do you have air in the tire or not. I think just what I quickly thought of and what MTSk&p mentioned, makes 64 different combination of ways to apply inside prep.
Mostly maxxis Ht3, going to start experimenting with some yellow vega(s) before long. We are running on packed red clay in central NC. I am just recently looking to take on tire prepping on my own.