Often when I pour a sample to be measured, there is a good 1cm of head obscuring the SG value. It can stay around for quite some time.
Can I leave it overnight in order to take the reading the next morning without some mysterious (ie unknown to me) process changing the gravity giving me an incorrect reading?
If you're talking about a sample of wort before you've pitched the yeast, then I think the only thing to skew a next-day reading would be a wild yeast or bacteria.
If you're talking about a sample of fermenting beer, then the yeast will be making the gravity go lower while the sample sits waiting for you to read it.
Personally I use a small graduated cylinder to take readings. You fill the cylinder to just below the top and then when you put your hydrometer in, a small amount spills over but the hydrometer reading and top of the beer level are all at the top of the cylinder. (the slight spill over gets rid of foam) I will also then simply drink what is in the cylinder for a taste test as well.
Overnight, you would probably see a slight increase in gravity in a wort sample, due to evaporation of the water (and thus concentration of the sugar). I don't think that's enough time for any wild yeast or bacteria to get enough of a foothold to alter the gravity significantly (which would be downward pressure, as opposed to the upward pressure of evaporation).
You could cover your sample glass with some sanitized plastic wrap or use a canning jar with a lid though, which would probably prevent both things that might alter the gravity.
If the foamy head is the only thing that's bothering you about the sample, then I suggest rubbing a little oil, Fermcap-S, or dish soap on the inside the sample cylinder. After all, we spend a lot of time figuring out what messes with the head on our beers, so we might as well use that knowledge of how to loose head to our advantage!
