Thanks for your reply. 

Re imagery: Of course I think that there are inaccuracies when looking at the imagery used for mapping the footprints. I you view my changeset one of the buildings differs by a factor of two, though most are probably a few tens of %.

Re precision: I can only go off of what I know and see, so I wasn't sure you were aware of the concepts I commented about. I am not advocating for mm precision or anything like that, just that the footprints be equal to, or slightly smaller (90-95%) than the roof giving a range sizes I'd consider being as accurate as the imagery allows. 

As far as time goes, I figure it shouldn't make a big diference, at worst. At best it may save you time especially in dense areas, or if there's a lot of existing data. Personally this approach saved me some time, becasue in the past I would use the vertecies of roofs to aquire a buildings' shape and size (like shown in the vast majority of tutorials/guides), but that leaves little wiggle room. You may actually find that it saves you time too.

- Best