A question I've always wanted to ask but kept forgetting.

Makes no difference - the amount of expansion or drain back is tiny. Just make sure you check on a reasonably level surface.

ordinary hot car, just a couple of minutes to let the oil fall back to the sump

The message from "Dave Plowman (News)" contains these words:

I'd say it varies from engine to engine. I had an old Fiat once that showed seriously low if you didn't wait five minutes.

Flat engines certainly do - well, all mine, anyway. The recommended period to wait for a VW engine is overnight!

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Alex Buell saying something like:

If that ^^^^^^^ has anything to do with it, I'd say you waited too long.