I have done lots of batches without taking on OG. I figure it will still come out beer without one. Punch all the ingredients into a recipe calculator and get a pretty close expected gravity.
To me the FG is what you need to take to see if you are done, waiting or stalled.
I do tend to not get too scientific though and have done batches where I never wrote anything down or measured (except temps) anything.
All that said, most of the time I do take an OG and I'm up to three hydrometers now.