I have been in mid-January, late January, early February, mid February, early June, early September, mid September, late September, early October, mid October, and early December.
Of those times, the least crowded has been late January, early February, and early September. The most crowded was June, of course, and then early December.
It's comparatively crowded over MLK weekend in January; it gets more crowded towards the end of the week before President's Day in February; and we felt it was crowded when we arrived at the tail end of Columbus Day week in October (it was dead from Sunday on, but Friday and Saturday were pretty crowded, IMO). Late September into early October is crowded when compared to early September into mid September (but still, overall, a pretty nice time to visit crowd-wise). Also, you have to keep an eye on Mardi Gras (there was a pretty noticeable influx of LA residents when our trip coincided with a February Mardi Gras date).
All of the times I've been, other than June, haven't been very crowded at all. I think they'd all be considered uncrowded times to visit.