It depends on what you think the primary purpose of grading is. Some see it as a way to show who has mastered the material, and some see it as a way of distinguishing students from one another. The former would likely prefer the vast majority of the class getting an A, if the vast majority mastered the class material; while the latter would likely prefer only a fixed percentage of the class always getting an A to distinguish the ones who have best mastered the material.
In either system one can use a curve and challenging tests. For example, the grade distribution for the Stanford electrical engineering class ee101a is pictured at http://www.stanfordrank.com/ee101a . This is a class that is almost entirely taken by EE majors at Stanford, who tend to be excellent students of which the vast majority are expected to readily learn the material. So the class is curved such that more than 2/3 of the class receives A’s, and there are no grades below B-. Had the class did unexpectedly poorly on the exams, the professor has the option to use a different curve with a larger portion getting B’s/C’s or lower. In short, the curve was not fixed (or at least was not when I took the class) and instead can be set up to reflect the percentage who master the material. Also note that there were very few A+ grades. It was my experience that A+ grades were often reserved for something really exceptional beyond just mastering the material. For example, I received an A+ in this class primarily because I found some additional solutions on a key exam question beyond just the ones the professor had considered. This can be a way of distinguishing students in such a system.
There are other complications as well. Princeton tried to have a system where a fixed percentage get A’s in each class, then abandoned the policy recently. The internal report that likely led abandoning the policy talks about the majority of students saying the policy negatively impacted their experiences at Princeton; In the first class some professors saying something to the effect of “There are x students in this class, so only y of you will be getting A’s”; Some students refusing to collaborate with or assist classmates to increase the chances of being one of the y students at the top of the class who gets an A; Some departments completely ignoring the guidelines and still giving the vast majority A’s; Students believing they are less likely to be admitted to professional/grad school or get jobs/internships than screen GPA; Students listing it as reason to not apply or favor HYS… over P… For such reasons, it’s common to err on the side of high grading. A’s have often come to mean more than just mastering the material. A’s can indicate just have satisfactory understanding… essentially A = Average. This degree of grade inflation is also a reason some cite locking grade inflation with a fixed percentage get A’s system.