This is an evolving question these days, but generally the starting point is what your head of sales owns.
Most Classic VPs of Sales really just want to be closers and own new bookings. Even if they want to retain accounts for upsell later, they still just want to be farmer-closers here.
Making a Classic VP of Sales also in charge of NPS, CSAT, and Net Negative Churn is a recipe for a suboptimal outcome.
But … if you have a VP of Sales (or CRO or whatever title) that wants to truly own the ARR number at the end of the year, all-in. Then — she or he really will want to own customer success, too.
Later, as you scale, this will become more specialized, and the line between customer success and account management will become more distinct, and these different functional areas may report to different people.
But up until $10m in ARR or so, I’d sugest have her report to whomever owns her KPIs. If the VPS own new deals, have the Director/VP of CS report to the CEO, or the Next Best Choice. If the VPS owns All ARR — then see if the director of CS can report to her.
Nothing is 100% perfect here.