I typically dryhop 5 -7 days before packaging, so I don't usually check the FG until then. I wait until primary is done, and the beer is starting to clear (usually day 10ish) then add the dryhops and then package 5-7 days later.
If you don't wait until fermentation is finished, the hops oils can "cling" to the yeast, and when the yeast drops out, the hops oils can go with it losing some of the dryhopping aroma. It might not make a huge difference, though.
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