If I took a reading sample right at flameout, no. But after cooling started, readings usually seemed more erratic. I attributed it to wort stratification. Nowadays I just take a preboil refractometer reading (actually a couple minutes into the boil to get good mixing) and a hydrometer FG reading, and call it good. It's by no means the only way to get there. Just my routine.
Since I've heard this for years, I've tested it several times and never seen it happen. Readings immediately post boil from various depths were the same as reading 15 min. and 1/2 hour later. Now, that's iust me, which is why I'm looking for evidence that might contradict what I've seen.
And I didn't do any proper experiments, Denny. Just mostly observations. Like for example, taking a flameout reading (hydrometer sometimes, refractometer sometimes) that lined up with where I should be with my preboil reading, and then took another after cooling that was several points off (with accurate volumes). So honestly I just switched to the preboil and FG thing, and it's habit/routine now.
Edit - And of couse evaporation after cooling would change the reading a little. But I'm talking 10 or 12 points difference sometimes.