[KTL]

I had an adjusting screw on one of my rockers in the '87 that went funky. You could only turn it in so many turns before it would bind. That's an example of what happens when you over-jam the jam nut.

Plus, as mentioned by Walt, when you overtighten them you can't get them out of the rocker arm when the rocker is removed from the engine. It's either the threads got booged by the jam nut or the slot in the adjuster stud has expanded the width of the stud. So you end up popping the adjuster foot off of the stud to remove it from the rocker.

Snug/mild resistance is plenty good enough for these jams. Gorilla tightening not required!