I have only done limited step mashing so I am not a good reference there. Generally, I have held temperature at a step long enough to give the enzymes some chance of doing what I expect them to do. A 5 to 15 minute rest below saccharification rests is pretty much all that is needed for most purposes. Once you get to the saccharification rests, where the activity of the alpha and beta amylase enzymes are maximized, I base the rest upon the amount of fermentable sugars I want versus the body of the beer from the dextrins which are produced. In general, the time you spend above the solubilization temperatures of the barley starches will give you the best chance at maximizing the gravity you extract from the grains.
That said, the main obstacles to getting more gravity from a given mash will be more dependent upon the crush of the grain, the types of grain used, infusion and sparge volumes, efficiency of your sparge, and mash pH than it will be with how much time is spent at what temperature when within the saccharification range. The alpha and beta amylases both perform different activities on the starch molecules giving you either longer random chains of dextrins (alpha) or consistent production of maltose molecules (beta) but neither one will produce consistently higher initial gravity from the mash.