There are quite a few exceptions to the rule...
For instance, I currently fly quite frequently from a grass airfield that has a slope. Although very mild, some of the "higher performance" / tailskid (without brakes) aeroplanes would prefer to land up the hill whenever possible, not only is the club house up the hill, but it is easier to stop going uphill rather than going downhill. Conversely some lower powered aeroplanes prefer to takeoff downhill and land uphill (due brakes or other). So that gives you some "variable" circuits which may make someone un-familiar with the airfield uncomfortable. But with good communications, and accurate representation of where you are, it is quite simple to slot in where you think you should, and if needs be, climb / remain in the overhead until the particular traffic has landed.
Tricycle gear pilots however, rarely notice the slope at all, and makes little difference to them...