Do we need to routinely monitor BMD in patients on bisphosphonate therapy?

Is it helpful to routinely monitor bone mineral density in patients who are being activley treated with bisphosphonates? If so, do we need to check both hip and spine BMD or would either of the two be suficient on its own? The National Osteoporosis guideline has no recommendations on this yet but this study provides useful evidence to guide future practice.

http://www.bmj.com/cgi/content/full/338/jun23_2/b2266