this is more an academic / theoretic question:
is it ok, to layout the CV s of a surface, so there is a corner with 180 degree ?
example Surface has 6x6 points …

It’s considered bad. At that corner, the surface is “undefined,” you’re dividing by zero. I avoid at all costs.

Some people making for example boat hulls nonetheless do this, of course by the time you’re ready for production there’s going to be SOMETHING along that edge, no matter how small, that trims it off.

As Jim said this is a common method for making untrimmed three sided surfaces without a zero length side, particularly boat hulls. Boat hulls with smoothly curve stems - how to model - #8 by oliver.jans
Problems arise with extending the surface because both parametric directions are parallel to the edge at the singular point. Therefore the usual methods for extending the surface fail at the singular point. It can also be difficult to fair the surface at the singular point.

Hi Tom - where U and V come parallel, the surface normal is impossible to calculate - offsetting and fillets (which use offsets) become impossible. It is legal, but to be avoided in general - sometimes you can arrange for this part of a surface to be trimmed off.