Make a specific tag the default window in OS X Finder
As you have tried, Smart folders/Saved Searches by themselves can't be used as the default location.  However, they can be made into Canned Searches, which you can use for new windows.

Here is what you need to do.


Create a new folder called To-Do (which will become the Canned Search) into your Documents folder.
Switch back to your home folder
Run a search with the To-Do tag as the query and the Search value to either "This Mac" or your home folder.
Click the Save button in the upper right corner
By default, it will be showing the ~/Library/Saved Searches folder.  Instead, switch to the To-Do folder you created in step 1.
Save the search with the name search.savedSearch
Open your Documents folder
Select and Get Info on the To-Do folder
Expand the Name &amp; Extension section
Rename the folder from To-Do to To-Do.cannedSearch and press Return
Enable the Hide Extension checkbox below, which should become available after renaming, then close the Get Info window.
Open the Finder menu and choose Preferences
Click the "New Finder window show" pop-up menu and choose "Other..."
Browse to your Documents folder and choose the "To-Do" entry
Close the Finder Preferences window


Now whenever you open a new Finder window, it will show your To-Do search results.

Hope that helps!