I dont test for it but have the seachem additive. That being said i havent dosed with it due to not seeing a problem. Seems my wc replenishes it for me. Had some hammer heads falling off a long while back and that is when i grabbed it - someone said that couldve been a part of my problem.?.
IMO, it is fairly clear that strontium is not needed by most organisms we keep. It was thought important years ago, but many, if not most, people who have kept reefs for a long time no longer dose it.
I haven't added any for at least 15 years. I've also not tested in that time, except that my Triton test recently showed it was depleted (as expected) in my aquarium.
It gets depleted because strontium can slip into the calcium carbonate structure in place of some of the calcium. That effect is largely uncontrolled by anything except the chemistry of strontium and the temperature.