Once a month seems to be a good choice.
I'll do it once a month and also occasionally at any given point during the month when I'm feeling like I've had some progress and want to check.
Personally, I measure "bra off," but when I was on Jenny Craig and getting measured by others, I was always in a bra (never leave the house without one one--people could get hurt!!).
I figure, as long as I'm consistent in HOW I measure every time, that will show me what my progress is. Since I'm not generally "sharing" my bust measurement with others, I don't worry about it being "right" as much as I care about it being a good indication of my progress.
Hope that makes sense!!