I see a lot of large accounts, I mean 10K and more, with no hashtags under their post. Is there a rule to it?
No. All i can say is they’re missing out big time. 
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Once you hit 100K+ followers, especially if you are in a niche, hashtags are less interesting because Instagram knows where to expose your content, within your followers and out there. That’s the reason why it might not be worth using when anymore. I do think a few hashtags help anyway…
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