Acer,
Good question!! Obviously, for the lollipop type sights we are discussing, angle is a moot point because it's most likely for a roundball gun which might never be shot further than 300 yards, if that.
Long range is different. For instance, one might initially want to shoot paper at 100 yards to get the sights with an initial zero as a starting point. The long range aggregate at Friendship consists of silhouettes at 200 -500 yards, a paper target at 300 yards, and a Creedmore target at 500 yards. The World long range matches are shot in 3 stages: short range 200-500 yards, mid range 600-900 yards, and long range 1000-1200 yards. Distances may be off a bit, but you get the gist. The same rifle is used for all courses of fire.
Since the eye is closest to the rear tang, some compensation of head placement will compensate for the changing angle. However, as distances extend, the cheek comes off the stock as the head is raised to see thru the sights. This is where interchangeable apertures in the front sight and the Hadley eye cup come into play. They are sized so the target is framed within the sights, showing only a bit of white around it for centering purposes. This helps compensate for the angle you asked about and still be able to see your target.
The base of the sight is designed so there is a very slight forward tilt to the staff. Lay a straight edge along the top flat in the picture and you can see this. But the higher you go, the difficulty becomes seeing thru the front aperture, because that is where the angle will affect sight picture. The man who shoots the rifle pictured has his front sight dovetailed into a base which tilts slightly upwards and this compensates for that problem. Unfortunately, I do not have a picture of it. These guns also kick like 10 mules, so the detent for the staff is spring loaded and it flops forward under recoil to prevent poking your eye out. It is then a simple matter of flipping the staff back into it's detent for the next shot.
Long range is shot in the prone position, either using X sticks or a wrist rest, but not both. It may also be shot from the back position, in which case the rear sight would be mounted on the buttplate instead of the tang.