I understand you want to know how to change the default slope used when sketching roof elements in the 2011-2013 Revit Architecture software.
Since the Slope parameter is unavailable prior to drawing the roof Boundary Lines, I don't see a way to change the default value.
Once you have drawn all of the Boundary Lines, you can change all of them at one time by clicking the Modify command, and then changing the Slope value listed for the roof properties (instead of the individual Boundary Line properties). This will apply the value you specify to all of the Boundary Lines at the same time (although it does not change the default slope value for new roof elements).
If you would like to see the option to change the default slope value in a future version of Revit, the following link is setup for you to submit feature requests, or feedback, directly to our Development group:
I encourage you to use the feedback link as our Development group is always interested and would like to hear your feedback directly, as you are better suited to state the business case for a feature request.
So.. as mentioned above if there is something you want Revit to look at send them feedback! I did, I also included the fact that I want views to be modeless so I can drag the view onto another screen...
To follow up: I just received a nice email from "Subject Matter Expert" for Revit at Autodesk stating that he will share these ideas with the rest of the REvit team for Consideration.... now I know Im not the only person asking for this but it's nice to get a positive responce from Autodesk...