I only use it like 10% of the time clinically, and generally speaking, time spent administering questionnaires is not billable. I'm never that desperate for that particular information clinically. And if I'm doing it forensically, I get to bill that time spent at a decent clip. As for validity, theoretically, it shoudl not matter if they come back another day to finish it. It's supposedly a measure of trait, rather than state characteristics. Chances are, if they'd invalidate it due to two separate sessions, they'd invalidate it at a very similar incidence in one contiguous session in most circumstances.
I'd never do it forensically, because the opposing counsel is likely looking for any meaningless break from standardized administration to throw the results out.