If you are simply zooming so the text in your editor is bigger, then why not just increase the font size?
Open TOOLS -> OPTIONS, under the ‘Environment’ tab select ‘Fonts and Colors’, and change the font size to whatever you want.
–edit–
As @Anovative pointed out in his comment and answer below, there is now a way to set default zoom within Visual Studio 2013 (and subsequent versions I’d assume).