Wear will only vary around the circumference in rather odd cases, e.g. a significantly out of true wheel with brakes that don't self centre, damage, or local contamination reducing friction.
The two sides should wear evenly as well, as brakes rely on squeezing the pads against opposite sides. Here though, it would be possible to get some uneven wear. Perhaps chain oil on the drive side would reduce friction and wear, or one pad could get a piece of metal or grit embedded and grind down one side, but you should hear that and fix it even if you don't do much to care for the bike.
Radially, an old rim will eventually have a taper both due directly to wear on the braking surface (but not right at the bead) and indirectly as the worn rim is thinner and deforms due to tyre pressure. If you're truing such an old wheel to a really precise level, you'll need it to be good and round, and well centred, or you'll be measuring at different radii where wear is different. But if you're seeking real perfection from a well worn wheel, perhaps it's time to go shopping for a new one.