I agree.
You have to have a steady partner to practice and improve with to best learn the footwork and technique for each step, as each person will feel different.
You also should not limit yourself to just that one person, as dancing with others will help the leader to fine-tune and clarify his leads, and help the follower become accustomed to different leads for steps. My partner has said some guys she has danced with will give such ambiguous leads that she doesn't know what exactly they're asking her to do. Others, though, are able to "put her where she needs to be" as she phrases it, and she knows just what is being asked of her. I've tried to learn the steps in our dances to get to that point, and so far it seems to be going well.
As part of our training, our coaches have assigned us to instruct the absolute basic beginning class, and that has helped us greatly. When you have to take the knowledge you know for a given step and actually describe it so that someone else can do the step, you end up learning something about the step yourself. It has increased our overall understanding of the dances we've instructed so far, and I get personal satisfaction when I see someone we've instructed getting a step just right and being able to dance it at a party. In our most recent class, we worked out a simple cha-cha routine consisting of several of the most basic steps. One of our couples had actually practiced the steps and was able to do them well, and they were actually able to enjoy the dance. I'm proud to say they've already signed up for next month!
I think you should have a third criteria, that of knowing both parts for a given step. Whether or not you plan to become a teacher, you should know both parats to complete your understanding of the step: where both people should be at any given point of the step, styling/technique, etc. Many times during our lessons my partner will feel like she is doing something or being somewhere she shouldn't, when in reality I was doing something wrong to make it feel that way. Our coaches show us how it should feel for the other person so we know how to do it properly.
...or maybe we're just overly studious dancers, lol
j