I think we need to keep in mind the source of this 75-percent number. It is basically a single rule-of-thumb for people to keep in mind so that they don't get too hung up on particular numbers. Years back it was fashionable to people to boast on their systems based on how high their efficiency numbers were. You don't want people to think that because so-and-so gets 85-percent that I need to keep sparging until I get that too. It is all very process dependent and hard to encapsulate into a single number, but pressed to do so, it probably isn't a bad number to use. However, in using a single number, it is important to keep in mind Kai Troester's observation from his 2010 NHC presentation on efficiency, that one man's 75-percent is not the same as another: one could have 95-pct conversion efficiency with a 79-pct lauter efficiency, while another could have poor conversion (79-pct) and over-sparge (95-pct lauter efficiency) and make a much different and probably worse tasting beer.
The best quality wort comes from the first runnings. As soon as you add any sparge water, the wort decreases in quality. For those of us who sparge, we count on the fact that there is a certain amount of sparge water that you can add and still not notice any significant decrease in quality, which is where other rule of thumb numbers like stop sparging when your runnings fall to 4 Plato come from. The 75-pct number captures this point by suggesting a compromise between the best wort quality and efficiency number. One may quibble about whether it should be 80-pct, 85-pct, or even 70-pct, whatever it is, I think as an all-encompassing number it probably isn't bad. It would probably be better to go the next step and come up with rule-of-thumb numbers for no-sparge, batch sparge, etc.
One is probably best served by making sure they get decent conversion efficiencies, at least in as far as it doesn't affect your lautering ability (don't grind all your base malt into flour, for instance). If you're mashing in your kettle with a blanket wrapped around it, maybe this means stirring your mash from time to time to get a uniform temperature. Your lauter efficiency will be what it is for your system and your process (batch vs. fly vs. no-sparge), and if you change your system (go from braid to manifiold to false bottom) you'll need to figure out what your new lautering efficiency is. At the homebrew scale it isn't nearly as important to have a high efficiency as it is to have a consistent efficiency.