You aren't the first person I've heard take this stance. Sheldon Brown states "In almost every case that I know of where someone has tried to adjust the tension with this nut, the saddle has been ruined. My advice is to leave it alone."

However, this is sort of senseless to me. Leather necessarily stretches over time with load. I'm not a big guy (145#), but I ride 5-6000 mi/year. When I start feeling like the saddle is no longer taught enough to carry my weight through my sit bones, I tension it. I've done it twice. The first time was after probably 3000 miles, the second probably a slightly shorter interval, but I made a much smaller adjustment that time. The first time there was essentially no resistance on the nut for the entire first 2 revolutions of the nut. After doing a bit beyond that, the saddle was markedly more comfortable.

I should state that I follow the 'letter of the law' when it comes to Brooks saddle maintenance, using proofide in the appropriate amounts, techniques, intervals, etc.

I'm not sure why this prudish approach to saddle tensioning is so prevalent. That being said, I'm not at all confident that I'm doing it the right way.