I'm not sure but I think jwilliams4200 is saying just test it once and if the SSD turns out not to be the bottleneck then fine don't test it again. At least we know that is something not bottlenecked by SSD performance. Is there a way to just time Windows boot to desktop not including post? Because as you wrote post varies greatly from board to board.
Well, even if it does not vary much between SSDs, boot speed it is still a worthwhile test. If there are a number of common tasks (that many people do) where the speed of the SSD does not make a difference, then that should be represented in the review testing. At a bare minimum, one of those common tasks should be included in the tests, and preferably more if there are a lot of tasks like that. Otherwise, the review gives the wrong impression about how important SSD performance may be.
I think the boot test should include the POST time, but the review should also specify the approximate time required for the motherboard to finish initializing (easy to measure with a stopwatch), so that it is clear to the readers how much of the boot time is taken up by that process.