I don't have all the answers but I do know a few things. Firstly, trimming does not affect the technical grade except when differentiating the various uncirculated grades. In other words, a Gem Uncirculated note needs wide even borders, and if you take a perfect note, trim away most of the borders, then your note would grade Choice Unc at best. Circulated grades are not based on margin size or evenness, they are based on wear. What trimming does is affect the appearance of the note. Most of the TPG companies assess technical grade based on the parts of the note that are present, and that is actually the correct way to do grading. If you decrease a note's technical grade based on flaws, what you are doing, in effect, is called net grading, which is a fancy term meaning lying by omission. In other words, if you take a note that is VF, but you notice a few pinholes, and you call the note Fine without mentioning the pinholes, you are lying about BOTH the technical grade and the pinholes. People do this because they think they can get a higher price by net grading rather than stating the correct grade and the flaws (i.e., VF with pinholes). The logic is that a note that is net graded will look better than its stated grade, and in an online sales environment where buyers can see only the image of the note (because they cannot inspect the note in person), this fuels speculative buying/bidding, meaning someone will offer more for the note thinking they are getting an undergraded VF with no pinholes.
Now, on the issue of trimming, older series notes were printed on sheets that were NOT cut prior to being sent to banks for issue. I don't know exactly when this practice stopped. The Charlton catalogue is rather vague on this subject. But what this means is that notes were cut by hand when they were needed, and as you can imagine, humans cutting individual sheets into single notes produces a large variation in border size and straightness compared to machines cutting stacks of sheets. This is another reason why the value of older series notes are more greatly affected by appearance compared to modern notes. This has the seemingly paradoxical effect of having mid-grade notes with nice jumbo margins selling for more than high-grade notes with tightly-cropped edges or some of the design missing due to a crooked cut. But that only seems paradoxical to people who insist on believing that technical grade = value. This is why really ugly-looking notes tend to stay in TPG holders. Notes that look nicer than their technical grade tend to be removed from their TPG holders. Ugly notes stay in these holders and the owners try to get a price based on the technical grade alone.
So, again, I will state that most older series notes that were cropped into their designs are probably NOT trimmed. They were probably made that way, but not on purpose. The people 100 years ago didn't create poorly-cut notes for a reason. It just happened. For the collectors 100 years later, it's really just academic if a note was badly cut in the past or given a trim job in more modern times. The result is the same. The value of the note goes down when there is a portion of the design missing. That's why I have a really hard time understanding anyone who would remove a note's margins in an attempt to hide some other flaw. But then again, there are people stupid enough to bathe notes in strong chemicals, like bleach, and then dry the note without removing the chemical smell! Do these people really think there are many collectors lacking their olfactory sense??
With shinplaster notes in particular, they were not always cut prior to being issued. Full and partial sheets were sometimes folded up rather than cut, and some survive today in this format. That's why you will see occasionally at major auctions sheets of shinplasters in circulated grade with folds. I don't know if it was common practice to do this, but it happened.
Measuring the dimensions of older series notes is completely pointless. This practice is fueled by a few people harboring an intense paranoia about buying trimmed notes. That's why my advice is so practical, and I'll state it one more time: "If a note looks trimmed, it's trimmed". But I guess some people feel they lack the ability to judge a note's appearance and need something tangible on which to base an assessment, so they pull out their rulers and try to collect only notes that meet or exceed a certain size. Of course, these people don't know what to say when you show them two notes of the same series where one note's printed design is larger than the other's... Notes were printed wet and then dried to different sizes. That's one more reason to take my advice.
In any case, all notes that become altered lose their value rather than increase their value (as stated earlier).
That's true if the alteration is obvious. When it is really professionally done... never mind.