when i first began looking at histograms they messed me up a bit. that's because i had a lot of video editing background and i was (wrongly) equating a histogram with a video wave form monitor. the representations of luma levels on a wave form monitor actually show the luminescent peaks and valleys of the image you 're viewing, i.e., from black to white.
histograms are very different. they show a frequency distribution of the luma levels. in other words, you're not looking at where the black and white levels occur in the image but rather their level and distribution throughout the image. and when you're shooting against a black BG or a white BG the histogram may not be so user-friendly when trying to decide if you're properly exposing the image.
histograms are my back-up check for the exposure levels within an image. first and foremost, my meter tells me what I need to know. i've also found that the LCD screens on the back of dSLRs are usually set to drive the luma levels brighter and this can be deceiving. Even when I turn the LCD's levels down, they still seem to be showing me a brighter image than I'm actually capturing.
i sometimes use a black/gray/white card, but not very often. when i'm adjusting levels in post i find that the blackest part of the eye usually yields pretty good results in setting black levels. i then adjust white and gray to taste.
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