I need to reply to this, and while in 99% of the cases, you're right, there is one time where this rule is not correct.
If you own a highly tuned competition handgun of the 1911 variety, you SHOULD KEEP THE TRIGGER DEPRESSED WHILE DROPPING THE SLIDE. Now, the most important thing to do here is make sure that the handgun is pointed safely down-range... But this is an example of having your finger on the trigger without having the target in your sights.
BTW, the reason for this rule is that many highly tuned 1911's, especially those with very light trigger pulls (mine has a 1.5 lb pull, and I've seen some as light as 12 oz) have carefully honed bearing surfaces on the sear and hammer. If you just drop the slide, those surfaces can get battered, eventually causing ADs. If you have the trigger depressed, the disconnector takes the pressure off of those bearing surfaces. When you fire the weapon, you can't release the trigger fast enough, so the disconnector is engaged there too...
I got this information from Ron Power, master revolversmith, and former armorer for the US Army's AMU.
Mark
I would never own a weapon with a trigger pull of less than four pounds. It is too dangerous.