I always use a strap with mandolin, banjo and Dobro, standing or sitting. Part of the reason is my lower back does better if I do, and for the same reason I also tend to sit while playing whenever possible, especially with the heavier instruments like banjo and dobro.
But I don't think there's a firm rule. I think a person needs to try it both with a strap and without, and for that matter, both sitting and standing, and determine what works best for them and their instrument(s). And don't be too quick to judge other people about sitting; if you haven't had LBP yet, you probably will someday.
(I also never use a strap with my double bass, but I do play double bass sitting on a high stool whenever possible.)
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