It is almost impossible to strip a bottom bracket by over tightening. I use wrenches about a foot long, and almost all my strength so it's in excess of 100 foot pounds of torque. On your cartridge BB be sure to use plenty of grease or anti-seize on the inside of the cups where the cartridge sits.

If you feel you've been tightening enough take a moment next time to feel for radial play in the threads before you tighten. This used to be rare, but slightly oversize threads in the BB shell have become more common over the last 10 years or so. If the thread is sloppy the rocking action from pedaling on both sides can start to work things. Solve it with filler in the threads to take up the slop. I use old fashioned fiber dental floss carefully wound around the cup for the full length of the threads and held in place with a film of grease. Then I force the cup in and it stays until I remove it.