Tee shapes can be orthogonal (rectangles, squares, etc...) or organic (kidneys, amebas...). When designing a landscape, it is recommended to have a consistency of vocabulary, if you want things to blend in well together. In other words, if you start with flowing lines, then use flowing lines. Don’t mix and match.
If you use a straight line in an English garden, for example (or a curvy line in an Italian garden), that line is sure to be noticed. Of course, this may not necessarily be a bad thing. In fact, it is a strategy often used in design, when you’d want a feature to be noticed, as a strong focal point of the overall work.
The curve doesn't blend in, but that doesn't make it bad. It becomes the focal point.
Unfortunately, tees are not the focal point of a golf course. And it is certainly not with this goal in mind that architects would give them orthogonal shapes amidst an organic landscape. Rather, tees are most often given a rectangular shape as a clin d’oeil to the older golf courses, where many features where built that way. That, and the fact that rectangular tees are still accepted on golf courses today.
Of course, we must remember that orthogonal shapes have, by definition, strong axis of direction. They lead the eye along their edges. And the longer that edge, the stronger the axis becomes, and the more important it is to have something meaningful at the end. This is even truer when the straight line your eyes follow is the only straight line around. As such, your eyes are naturally lead towards the end of a runway, or the end of a tee. If that tee points in an arbitrary direction – or, in other words, if there is nothing meaningful at the end of the axis- it is confusing visual letdown.
A straight lines “points” in a certain direction. It is important that there be something meaningful in that direction.
So if you, the architect, decide to use rectangular tees, then make sure they point down the fairway. If they point way off line, they become confusing eyesores. If they point only slightly off-line, it will look like a mistake or like you did a half-assed job.
It’s got nothing to do with golf. It’s just proper design.