Presumably it depends on where you live, and which stations you want to receive - they probably don't all come from the same transmitter.
It is the case here (in Warwickshire, with transmissions from Sutton Coldfield) that all of the BBC national, and most of the local stations I listen to come from the same transmitter (or at least the same direction). DAB is another matter, and seems to be broadcast from all manner of strange places.
The best thing is to try it and see - that's easier in the loft than on the roof. Bear in mind, also, that radio transmissions from the main sites (such as Sutton Coldfield) are mixed polarity - I found that my tuner sounded better with my aerial in a horizontal position. If you live near a repeater, though, I believe that they're only vertically polarised, so your aerial will need to be vertical.