"my S-I-L always trims his to the recommended trim length in the published manuals"
A lot of people happily do that and it works fine. It may be interesting to know that at the time I started reloading in '65 no one even suggested a "trim to" lenght but it was common practice to limit trimming to about 10 thou less than SAAMI max case length. There's nothing scientific or magic about that number, it was simply a reasonable trim measurement even tho we knew chambers could usually accept 20-30 thou longer. Most of us just used cheep aluminum "snap gages" to insure we weren't too long and many of us used a hand held fine cut mill file as a 'trimmer'. Then inexpensive precision calipers and case trimmers came on the market and suddenly all the new guys were antsy unless someone told them exactly what to do, so manual makers started publishing 'trim to' lengths but you may note it's still the same old '10 thou less than max' rule of thumb we used wayback.
The same sort of thing happened to OAL measurements. Few old data sources even mentioned OAL, it was something we knew had to be worked out individually based on the individual bullets and chambers (and still does really). A few sources gave the standard SAAMI max lengths but that was based on the length of magazines and feeding mechanisms, it had nothing to do with best for accuracy or safety! Then, since Chinese calipers at rational prices were suddenly available, a few of the load book makers gave the OAL that was used in their testing but knowing it only applied to that firearm. Suddenly it was presumed we had a supposed "reccommended" OAL and virtually every data source was expected to provide it. It wasn't much of a step from that to folks thinking the book OAL was the "right" OAL and differing could lead to injury or death. Not true by any means, neither I nor any other old hand even bothers to look at a book OAL.
