Rebs,
In the 1992 SAAMI standard, the length of the 223 case was 1.740" to 1.760". In the 2015 standard, it was changed to 1.730" to 1760". As a general rule, manufacturers will aim for the middle of the range to give themselves the maximum tolerance error range. By that standard, the trim-to length used to be 1.750" and is now 1.745", to match the length manufacturers are expected to target with new brass. However, I have not noticed any of the reloading manuals dialing the old trim length back to the new dimension. Nor do I know why the old dimension was changed, as no other rifle cartridge has that 0.030" wide window of case length that I am aware of.
As an aside, while the 5.56 STANAG documents are still marked as classified in the usual catalogs, it has been generally true from the 30-06 forward that military brass has only a -0.015" case length tolerance and variability window. The maximum case length is the same as the SAAMI drawing, but the mid-point and newly manufactured brass or trim-to length would be -0.0075" shorter than maximum. So if you want to use military ammunition specifications, aim at 1.7525" trim length. I am unaware of this making any functional difference over using 1.750" other than needing to be re-trimmed a little more often, but thought I would throw it out there as one of the possibilities.
The fact is a lot of M14 shooters used to trim their military 7.62 brass back about -0.040" shorter than maximum and -0.020" shorter than SAAMI minimum just so they would only have to trim it once in its reloading life. These were match shooters and they had no problem with the practice that I've ever heard of. They were not shooting the shortest bullets, though. This would have been for 168-grain MatchKings and not for 147-grain bullets, which often offer rather little seating depth of the bearing surface of the bullet when loaded to military standard overall length. But, in principle, the same should apply to 5.56/223.
Overall, I would suggest you simply pick a trim-to length you are comfortable with and stick to it. The book value of 1.750" is just fine.