Time management skills are important but can be developed in other ways like balancing courses, Ec's, research, tutoring, etc. This will add strength to your interviews and essays when you talk about how doing all of things prepared you and taught you to manage time. Saying I learned time management skills by only taking 16-18 credit courses a semester is not a good example in my honest opinion. Also I mentioned that you should not take more classes to demonstrate time management if it will negatively impact you.
To give you an example, during my spring freshman year semester I enrolled in 21 credits of classes because I wanted to do research which gave me 3 credits. I was over the 18 credit maximum and I though I could handle it because I was a good student. Well, as I went through the semester, I was spending 12 hours at my lab weekly, including weekends, and my grades in classes were going down. I withdrew from 1 class to compensate for this and ended the semester doing well in my other classes but I had a W on my transcript now. This was my only W I ever had and I learned from this situation and elaborated this o my secondary essays. So I wanted to give you some insight about the difficulty of handling a heavy courseload. You have to be realistic in your abilities and how much work you may have, if you are doing other things and not just coursework