In terms of plateaus and training routines...
Medically speaking, the body adapts not to a specific routine but more a specific level of tension. Switching routines involves generally a changing of exercises and sometimes a shift to lower, heavier rep ranges. Both of those things put new levels of tension on muscles and can thus elicit new growth.
But, say you've found a program you like, and you plateau. Do you have to switch to a new program? Not at all! Yes, the muscles adapt to the tension, but you can reverse the adaptation by taking 2 weeks off. HST incorporates this idea as strategic deconditioning between loading cycles, but it can be applied anywhere. It would work great with the 5x5, because the 5x5 is very similar to a low-frequency HST, in that you start light and work towards heavy. 2 weeks off would allow the light stuff to cause growth again, and you wouldn't have to switch from a favorite program.
-casual