Most Wing Chun schools will teach the Siu Nim Tao form right from the beginning. It is universally accepted in Wing Chun circles, that this is where the foundation of the system is to be found. It’s like learning your ABCs, or your musical scales.
However, it is also true that because forms provide no feedback mechanism, it is possible to do them incorrectly for years and not realise it. What do I mean by feedback
Well, if I pick you up and throw you on the ground, your weight provides me with feedback. If I’m doing it incorrectly, I will know this right away by the pain I feel in my back, or my inability to pick you up at all. I will then refine my approach by doing it over and over again until I can throw you competently and with no pain.
This feedback is essential in the early stages, because as already mentioned, without it, one can stray from correct technique without even knowing it.
This is why in Judo, Kata is only taught once you are grading for your Blue Belt. By this stage, a Judoka has spent years practicing throws and sparring. The basic bio-mechanics are already there, and now, they can be perfected through Kata if need be.
So there is a strong case to be made that forms should be taught only after the foundational bio-mechanics of the system have been understood and trained through drills and sparring.
Only then are you sure not to stray too far away from proper technique.