If you can't test for it, don't dose it. That being said, trace elements is a bit of a broad term, and most of them can't be tested for. The only things I would worry about ever dosing are the big three, magnesium, calcium and alkalinity. Magnesium should be taken care of mostly by water changes, but testing for it occasionally it's good practice. Alkalinity and calcium can be taken care of by part a and b chemicals. If you try to go beyond that, you're really only complicating your reef keeping and dosing things with no real proven gains.
Michael Rice
Marine Engineers Aquarium Blog
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