Is it good courtesy to leave the bed made when checking out of accommodation?
No, in fact that would be bad etiquette.
You should not make the bed.
No more than you would, say, at the end of a restaurant meal suddenly get up and wash the dishes. As well as being somewhat bizarre, it would be disrespectful inasmuch as it would somehow imply that you think the service you paid for (provision of perfectly clean dishes) cannot be done properly by the staff.
Further, as Aaron notes, it's more systematically hygenic, for the reasons Aaron explains, to not make the bed when you check out. Quite simply the sheets are about to be taken away to the wash company who rinses them - why would you make the bed?
Also (I believe nobody has mentioned this so far), in the case where you are staying for more than one night:
... these days hotels have the thing to increase profits where, to "help the environment," they don not automatically change the sheets every day. In such a situation (unfortunately this is the norm now), to indicate that you do want new sheets, you just strip the bed a little. (I pull up one side of the bottom sheet, and maybe pull off one pillowcase, so that it's totally clear the linen should be changed for new linen.) In some cases there's a card you leave, "please give me new linen today"; I personally also half-strip the bed as described, so that it's more certain.
When you finally checkout, to be polite just leave the room "reasonably orderlyæ.
I would say, leave it "dirty but orderly" if that makes sense.
For example, any blankets you've left laying around on the floor, just chuck them up on a bed. So again for example, kids always strew every pillow and blanket all over the room. As I'm leaving, I just pile everything on the one bed, so the room is approachable for cleaning by the staff: no need to leave sheets, etc, on the floor. Or with towels, I make one large pile of towels, say on the bathroom floor, so they're all together and easily thrown in the hopper.
I understand this is probably a minor point..
It's a major, significant and subtle travel etiquette question. Great question.
If I leave my bed un-made, is that rude?
No - as in the restaurant example which shows it more clearly, whilst you don't want to treat service staff as subservients, you want to allow them to do their job.
Or look at it this way: you don't want to imply that their job is so miserable that "you have to help them out, by doing it for them". Housekeeping is a decent, honest, hard-working and honorable straightforward job, and you should be pleased to be part of the system paying for them to do the job; let the staff do their job.
An extreme example, would you help the sushi chef cut - of course not; let service staff do their job, too.