I just did my '30 Coupe this summer. I agree. Do the roof first.
One of the things I liked about doing it this way was that I was able to inspect the underside of the newly installed roof. I ensured that there were no pointy ends of chicken wire where they may poke through my new roof; inspected for bad nails; etc. I also drove mine this way for about three weeks listening for rattles or squeaks, and took care of a few small things before I closed it all up with the headliner.
I drove it in hard rain and had no leaks. I didn't think to use a garden hose... I like that idea.
I found roof to be a two day job (being cautious and taking my time). Headliner I did in an afternoon.
Oh, one more thing, I also took the time to put in sound insulation before installed the headliner and interior, and was glad I did.
Good luck.