Quarantining fish if suspect tank already has ich

[mark]

Realize you QT for more than just ich but something I been wondering (sort of brought forward here again). As it's almost accepted that our tanks have ich, though maybe not showing, is it worth the effort to set up a longterm QT for a new fish?

More for discussion as not planning any fish but my case, seen some spots last year on a tang and nothing since but I'll assume my tank has ich (as it's never been fish fallow). If I was to get a new fish, considering stress, a NH3 spike, etc, is 4 weeks in a QT really worth to risk?