I used to have an experience where, having tweaked and prodded a group of photos to my satisfaction and exported them, I would upload them to Flickr and only then notice that some seemed overall too dark for my liking. Recently I found a way to eliminate this problem.
I don’t know how I noticed it, but it seems my judgement of overall brightness of an image is best made using the thumbnail in the Move/Zoom palette. I don’t have an explanation for why this is so, but it seems to work.
I know the histogram is available but that’s not an intuitive tool for me to gauge ‘the whole’.