I've always had problems when trying to asses what was going on inside that darn 255 bar in histograms, no matter which program I was using (or the LCD on my 30D). The histogram graphics in DPP RGB is among the worst in that respect, due to the lack of contrast and the histogram border line being flush to my display's edge.
Following my advice, DPP++ now depicts an additional, real-wide bar outside the regular histogram frame for each 0 and 255.
http://digitol.free.fr/doku.php?id=dpppp:screenshot
I'm curious to hear if anyone find this solution to be as earth-shaking as I do? 
