In practice, when a taper "bottomed out" at the back side of the arm, I never had any problems with it, so I don't ever even look at "taper length" other than upon actual installation, noting whether some real bolt tension is realized before the outward flares of the spindle contact the back side of the arm.
The simple difference between JIS and ISO is that the JIS taper is about two-tenths of a millimeter larger across, resulting in about 2.5mm less insertion into the arm.
Thus, JIS spindles should be considered 5mm longer overall (not on each end) than an ISO spindle of the same actual length.
Comparing spindle tapers is easy and informative! One can compare insertion depth of two different tapers by simply locking the caliper at ~13mm and then measuring how far that each different spindle's taper slides between the locked jaws!
I measure spindle end dimensions by putting the caliper jaws flush with a table and then putting the end of the spindle flush with the table, between the jaws, shown below.
This way, the measurement will be at the same 1/8" from the end of the spindle, equal to the thickness of the caliper jaws.
The numbers I get are about 12.7 for ISO and about 12.9 for JIS.
The ISO spindles include Campag cartridge bb's, Sugino Mighty bb's, Suntour Sprint and Superbe bb's, and with old Stronglight bb's being very close as well.
Campag cup-cone bb spindles are in-between, but are significantly closer to ISO than JIS, measuring about 12.78mm in most cases. So not equivalent to JIS in terms of insertion depth.
JIS spindle tapers are claimed by one well-known source to be a copy of old French spindles, but in fact it is the old Sugino Mighty and Superbe tapers which are more of a close copy of old French spindle tapers (such as used on Stronglight 93 and 49d cranks).
Everybody working on bikes should have one of these $10-20 calipers, and spindles should be checked if for no other reason than to verify what I have said here, so that best approaches are used when trying to fit a particular crankset to a desired chainline dimension.
I've used JIS cartridge bb's with old French cranks, but it is essential to consider not only the taper length differences but also the asymmetric offset of the spindles that you will choose from versus the original bb.
For the picture below, actual spindle measurement shown, keeping the measurement away from any rounded edges at the end of some spindles. This is because of the thickness of the jaws forcing the measurement to be that same distance from the end of the spindle.
And again, it is also possible to lock the caliper jaws and then measure with a ruler how far up the taper of two different spindles that the caliper will slide. This will directly compare the insertion depths of the different spindle tapers.
Another piece of advice that I can offer is to keep a few used, but not-so-good-anymore cartridge bb's on hand for test-fit purposes.
So much time can be saved by doing a simple test-fit, and the used bb's typically retain their (steel) taper dimensions much better than (aluminum) crankarms do over perhaps decades of use.