Not sure what you mean! (and doesn't look like anyone else is, either). Can you clarify?

Resin

pineresin, sometimes I myself am not clear about what I mean. The recurring question in my mind is, why the leaf tip? There is no special reason why it should not be the entire leaf margin. There are no specialized cells in this area. I have seen the drop of water at the tip of the tomato leaf and thought that maybe the cells at the tip of the leaf were like a membranous filter, allowing only water to pass outward while the 'filtrates' remained, concentrating and causing a 'salt scorch'. But the leaf transpires at other sites and the discoloration is absent therefrom. To be sure there are many conditions which give rise to characteristic tipping; by looking at the recent cultural practices one can deduce the probable cause(s). The tipping to which I refer is ubiquitous. There is no rhyme or reason to it.
But I continue to be nagged by the thought that there is an explanation. There must be!

I always thought that leaves had tips on them to deliver the water to the root zone and away from the stems and crowns where rot could set in.

But I've also been told over and over that botany has changed since I learned it so the fact that this explanation makes sense, may have very little to do with the actual function. You be the judge.

Now if you mean why do leaf tips get scorched instead of the rest of the leaf, then I suggest that on many plants, the tips are a bit thinner, and the sun tends to hit them harder. But the truth is, there are probably many reasons for "leaf tipping" as you call it. I don't think this is a botannical term, however.

Then again, maybe I don't understand your question at all.

Lainey

Most exposed to air, dryness, or wetness? Most likely to rest on the soil? Most likely to be landed on and sampled by insects?

If by leaf "tipping", you mean some form of pervasive damage to the leaf tip, the tip is thinner (as botanybabe mentioned) and has less support structures in it - leaf edges also have the largest surface area to volume ratio, so absorbs and dries faster, and the tip is "all edge".

Also, I'd say the tip is a smaller area, and damage you see there is more obvious than on another parts - "leaf tipping", if it's what I think you mean, may be partly perceptual.

Many thanks for your thoughts!
Here's what I have observed on plants grown indoors.
The discoloration:
is black and limp. Overwatered; 2 to 3 weeks to develop.
is brown and crisp. Dried out too long.
has an almost phosphorescent yellow band between it and healthy leaf tissue. A puzzlement!
most times follows the leaf tip shape . No clue!
It is an indication of my life ( as in 'get a life!') that I should be fascinated by this. If I could understand why the leaf tip tissue is so reactive to such conditions, I might be better able to interpret the cause of what I see.
I have thought that the tissue in this area is oldest and that Mother Nature in her conservative way is drawing down on moisture and nutrients before shedding the leaf.
But this does not explain why the flow seems to go the other way sometimes.