Oh, Gigi, you are playing my tune.
Honestly...because I'm between sizes and (I gather?) have an odd foot shape, heel slippage is a fact of life for me. The only shoes and boots that don't slip on me are my La Canadiennes. And some of my sandals. Actually, one pair of La Canadiennes slips, too, but that might be because they're older and I had to add an insert for plantar fasciitis.
Anyway...I'm not joking. Especially with boots and booties, (or pumps, but I don't even go there!) my heels always move up and down. Forget heel inserts -- they don't work.
For me, it's a question not of "if" but "how much" and is it tolerable. The other aspect your mention -- the instep fit -- is also key. If the fit at the arch and toe area is good and the instep area is narrow enough that it "grips" me a bit or at least follows the curve of my foot, and I'm not swimming around there (which sometimes happens) then I figure I'm good, even with a bit of heel slippage.
How much is "a bit." Ah. There's the rub. (LITERALLY.) If it's just a bit of up and down, and the rest feels stable, it's totally fine. I do take the precaution, even with socks, of wearing bandaids where I think a blister might develop on my first few wears of most booties.
Sometimes it's too much though and I feel my foot pitching forward as well as up and down or I feel almost as if I'm rocking. That's no good. It also does depend on the socks/ stockings I wear -- sheer hose will lead to more slippage/ rocking.
I've had some success experimenting with different types of insole. You might try an insert just for plantar fasciitis (at the heel) because it might lift (or cup) your heel in a way that makes the whole thing feel more stable. Occasionally this has worked for me. Sometimes it does the reverse. Sigh.
Ultimately, I've learned that some lasts just plain don't work for me. If the arch isn't in the right spot, I give up on it.
I will say that these Aquatalias below bothered me a bit in the way you mentioned the first year I owned them. I also felt they "bagged" on me a bit (despite the slim shaft -- it is stretchy and it seemed to sag somehow.) However, they're a great colour for me and waterproof, so no way was I getting rid of them. They're on their third season now. And this year, unaccountably, they are excellent! I don't know how to explain this but I'm delighted and as a result I'm wearing them more often.
ETA: I am either a 6.5 or a 7. I have to order Aquatalias in 7 because 6.5 is a bit short and narrow for my foot -- so this is partly why I get the extra heel slippage with some of their styles. An earlier pair of Aquatalias gave me no such issues.