I think it very much depends on the kind of work you are presenting. Don't force work that needs to be different sizes into a one-size-fits-all configuration, but by the same token, if the body of work makes the most sense all one size, don't force it the other way either, just to break up a rhythm. I don't think there is a single rule about image size in a gallery show. For me, because I tend to make photographs that work in a narrative fashion, I display them all the same size. If your images are non- linear narrative, put up what you feel is best for each image.
The 'photographers rule' about printing and framing to the same size comes more from a reasonable suggestion of economy than anything else- if you print, mat and frame to the same size consistently, you can recycle your mats and frames from show to show, and you don't have to reframe everything for each exhibit, and more importantly, you don't have to store dozens of frames and mats in your house. And if you hadn't noticed, framing (even when you do it yourself) is EXPENSIVE.