The convention is that we express the time in terms of the hour that it's closest to. "A quarter past nine", not "three-quarters to ten"; but "twenty to ten", not "forty past nine".
"Half past" is, of course, exactly in the middle. The convention is that we say "half past" rather than "half to".
Sure, there would be nothing technically inaccurate about saying "half to". But as Barrie notes, the purpose of language is to convey meaning. If you don't follow standard conventions, it is more difficult for others to understand what you mean. I wouldn't say that you should never break the conventions. But I would say that you should only break the conventions when you have a specific reason to do so, like when you are trying to emphasize something or make a specific point.