Converting a GROUP of images to DNG only works in Grid View. This is typically a last move done from the Develop module as per the directions of the official Adobe Evangelist Julianne Kost. Therefore, and for the sake of a better workflow, it should work from the Develop module.
I think most people are not leaving this step for last and doing from develop module. i.e. it's far more common to do upon import, or do a one time conversion of library, then do upon import in the future. Export after develop: yes. Convert source files to DNG after develop: maybe worth re-considering..
Yep, I do it at import, or after transferring to my master catalog on my desktop(when I am Laptop bound on the road. I can't conceive of ever using a workflow where I would pull this trigger from the Develop module.