It is recommended to draw beam at least centre to centre so that all the beams formed enclosed area. This is to reduce the chances of encounter “Failed to lay out in non-enclosed area” error when pointing slab at later stage.
Reason of seeing non-enclosed area error is due to small gap appear in between beam.
Some users tend to draw beam from inner edge to inner edge (refer picture below) as they worry the quantity might be double at overlapped area. In fact, TASC will do auto-deduction when it overlaps with another beam.