When I open a code file in a new code window, I press Ctrl+M,O to collapse everything there. As far as I know this can be done by default, without need to press anything every time. I think I did it once, but can't remember where was this option located.
This is possible. Go to the Tools menu, then select options.
Text Editor
\ C#
\ Advanced
The option is called "Enter outlining mode when files open." When outlining mode is enabled, your regions are collapsed by default.