Thank you for the picture and code-snippet provided.
I try to reproduce the same behavior locally using binaries from Q2 2015, but to no avail. You can see on the attached picture that our label and MS label are aligned as expected without additional margin. However I might missing some important part of your implementation and this is why I attached the test project I used to reproduce the issue. Please feel free to modify it in order to show us how to reproduce the described behavior.
Thank you in advance for your cooperation.
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Many thanks for your help. The problem was on my side. I should not alter default style for textblock and add margin to it, because textblock is used as a part of many other controls (including label).