640 Loctite is green, and it sure as hell ain't "low-strength!" It's a sleeve retainer and requires more heat than red (271) Loctite for removal.
I hate generalizations. Using red (271) Loctite on the threads of a Glock front sight is recommended by many sight manufacturers and keeps the sight from loosening up and getting lost. For those of you who buy into the "if you torque it right, you won't need Loctite" mantra, then you'll be happy to know that both Ameriglo and Brownells sell replacment front sights for Glocks.
I use a thin coat of grease on the receiver threads when torquing an AR barrel nut, the
correct torque spec for which is from 36 lbs/ft to 80 lbs/ft. I usually torque to around 50 lbs/ft.
I don't use Loctite on scope rings; I use Torx head scope ring screws, and, per Leupold, use a tiny drop of oil before tightening up the screws.
When I was installing a Smith compensator on a "ban" AR barrel, I applied 640 Loctite, per instructions directly from Mr. Smith hisself. Have had to remove the brake a couple of times, and had to use a propane torch both times. I can't think of anywhere else I'd use Loctite on an AR, EXCEPT for the overtravel and engagement screws on my JP Enterprises triggers.