The only way I know of, (and that I use) is as follows:
Open textedit and create a new document.
Add the following 3 lines to the file
Save the file as "somename.css". I called mine defaultzoom.css and saved it in Documents.
In Safari go to Preferences | Advanced.
In the Style sheet: dropdown box choose "other" and browse to the location of your somename.css file.
That's it, now webpages will open with the default zoom level you used in the file (I use 175% but you need to tweak it for your screen size). Also, I do run into situations where some pages don't render correctly. If that happens you can go back to Safari | Preferences | Advanced and change the Style sheet dropdown box to none until you are done viewing that paticular website. Then just put it back when you are done.