Depending on from whom you learn your Tai-chi, you might start learning the square form first then the round form; or you might only learn the round form. Some practitioners said the square form is not Tai-chi and, working like a robot, will not help learning the true Tai-chi form, which is the round form. I beg to disagree with this latter view.
Another interesting point is that since there is in existence of a square form, proponents of which have been figuring out various reason for its practice. As for better coach-student relationship, the more seems to be the better! Again, not all reasons for practicing the square form are valid.
What is square form and what is round form? Round form is normally we consider as Tai-chi, in which the movement flows smoothly from one to the next (see Tai-chi as moving meditation here). Square form is like separating each smoothly moving unit into a number of (say 3 - 5) stationery forms (each being stationary for a few seconds only) and joining them together by curve lines that look like straight lines (you now know why some people call it robotic, I mean if you put in your imagination).
What are the benefits of square form? Say round form is movement of a circle, square form is simply stopping, and mind-focusing, at, say, four points and moving quickly, and without much mind-focusing, at the joining curves. Doing it this way can lay a solid foundation when eventually one does it smoothly through the complete circle. On the other hand, without specific focus on the four points first, a practitioners may not be able to do a very round circle without himself noticing his own inability.
Another benefit of the square form is that it draws a practitioner's attention into the possibility of doing zhan zhuang (standing meditation) at any one of the many fixed points of the whole sequence. Assuming one is familiar with the benefits of zhan zhuang, one can immediately grasp the benefits of doing zhan zhuang at various Tai-chi posts offered in the sequence. Prominent late Wu-style master Wu Tunan 吳圖南spoke about how he, as a kid with weak physique, did zhan zhuan in all stationary posts of the sequence and gained tremendous benefits both physically and mentally.
For modern practitioners, it pays to put more attention to the square form (even for one who believes himself to be already proficient in the round form), as well as trying to pick out some posts to be used as standing meditation.