It’s always preferable to prove the accuracy of your video levels using external scopes as long as they’re downstream of your output hardware.
One common problem, as discussed in the BuZZ show of Jan 25, is the levels coming from Adobe After Effects. Video levels could be full range 0-255 in each channel (or 16 bit equivalent) instead of the 601/video range of 16 -235.
May be some useful information at
http://www.dv.com/features/features_item.php?articleId=196601411 (might have to set up a free log-in)
and for super technical background
http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html