Hi, Akalory!
As a Russian classically trained musician and the author of Soft Mozart I have an answer for you: the truth about good curled fingers and proper posture is an old myth of many music schools, including Russian.
I wrote an entire book about the subject and you are welcome to download and read it for free. The link is here:
http://pianolearningsoftware.com/collections/learning-aids/products/book-you-can-be-a-musician-by-hellene-hiner-downloadable_p_36-html But in short I would like to reason with your based on common sense that all of us (I hope) developed to this age
What is piano playing? It is placing fingers on ground (piano keys). It is very similar to walking, jumping, running initially before dancing and doing mord advanced tricks.
When we or our babies learn how to walk, they first learn to keep their balance, they 'try the floor' by looking at it with eyes and developing eye/body coordination. No one in the right mind would tell the babies how to fix their posture and provide some pa-de -deux, No one, but 'traditional' piano teachers.
They forse children to keep curved fingers and 'correct position' before they let them to crowl and develop fingers awareness and balance. This is insaine!
No wonder that NONE of Soft Mozart students ever develop bad posture or technique. They develop gradually with visual help.
A picture worth 1000 words and I want you to watch couple of short videos about this exact problem.
These videos were BOMBARDED by piano teachers who place a car in front of a horse and charge you $$$ for no effective and hurmful way that they provide to teach children.
They are doing it fo centuries and how many REALLY GOOD piano players they raised? Just few with inborn talent. How many did they destroy? Majority of population....
So, here is my rebuttle, since they almost destroyed a musician in me with these not so good tricks....