Maybe it's indicative of the kind of courses I play, but I can't remember seeing a tee box that wasn't square, rectangular, or "L" shaped.
What's the point of a round tee box? The straight line between the markers needs to be accommodated, and this is clearly achieved more efficiently if the front and back edges of the tee box are also straight. Why have have extraneous turf to the front and rear? It sounds like an affectation.
Maybe however, I'd change my view of round tee boxes if I ever actually saw one!
Edited to add:
Reading further around this subject, I get the feeling that rounded or free-form tee boxes tend to be used in situations where there are multiple tees. I guess that makes some sense. On the 100 year-old courses I play, there is generally one or at most two large tee boxes for the men with maybe a smaller one in front for the ladies. They are universally rectangular or "L" shaped and to my mind look "right".