Hi Ger,
Yes I would definitely have a fallback option! For me I setup my grid with all the containers I need then switch to Fallback First. I design the entire website in Fallback First that way I know that for older browsers or that stinking IE Explorer that it will look good. When finished I switch back to grid and slightly tweak it with the way I want the website to behave on all modern browsers.
If you try to do this backwords, ie set up Grid build then switch to Fallback First you will have to completely restyle the entire website.
It seems the way CC developed the app if you design in Fallback First to start with all of those styles carry over to Grid but not the other way around.
Hope this makes sense and helps.