Yes, I think I would.  

How tall (now) is the grass?  How tall (now) is the johnsongrass?

Depending on how tall the grass and johnsongrass is, I think spraying before discing would help to eliminate both.  Killing the grass before discing would definitely help to make the discing more uniform.

And killing the grass now would help to conserve moisture, so there is more moisture in the soil when you are ready to seed the alfalfa. 

I see a couple of good reasons to spray now.