Is using the supplemental lift on opposite days ideal or it really doesn't matter? For example On Squat 531 day using deadlift fsl and on Deadlift 531 using squat fsl.
It's not a bad option to get more frequent practice in the lifts and stave off monotony. Jim has it programmed in some of the templates in his books. I'm doing that currently and I enjoy it.
It doesn't matter. People are too busy looking at the week and not the total work done in a year.