I don't know that there's a single best practice here. Different woodworkers tackle certain things in different ways and we have a good example here of the differences in philosophy between one maker and another — the resistance to the idea of using a jigsaw for the cut in the first place. This despite curved cuts being one of the things a jigsaw is primarily intended for, coupled with the fact that they make blades specifically for wood, one could say that a jigsaw is a perfect choice for this job.
Hole-saws are obviously purpose-made for this too, but they aren't immune to tearout either. Especially on a fibrous wood like red oak.
With the right jigsaw blade used at the right speed you should normally get minimal tearout on at least one side of a board (depending on the blade this can be the entry or the exit side). With a particularly good blade such as the Bosch Extra Clean both sides of the board can end up equally free of tearout.
However, even if you are using a blade that won't give a perfect entry side, there is another way to help prevent tearout and that is with a zero-clearance plate or insert. This offers the same advantages as a sacrificial board, without the added cutting effort and the waste of the sacrificial material. The zero-clearance can be left in place for all vertical cutting, only removed for angled cuts.
I worry that I'll get better sanding results without the hole in the way
I think you're right to be concerned, but with just finish-sanding still to go you should be OK.
I would actually use a card scraper instead of sanding for this (faster, no dust, better results), but either way I would smooth the wood prior to cutting and then examine the hole edges, hoping I wouldn't have to but prepared to do touch-ups if needed.
I could have to re-do some sanding if the foot of the jigsaw scratches the surface.
I think your plan to tape the foot of the jigsaw with tape is a very safe way to go, done right even on softer wood like pine you nearly remove the risk of light surface scratches. But if you make a zero-clearance insert from a material such as hardboard which is softer than the red oak you will have even greater assurance that you won't scratch the workpiece.