To be honest for a long while I had never thought about aspect ratios. I always like you cropped the picture to what I thought was best(if it needed cropping of course).
But when I set out to prepare for a gallery I did in November and one in December, I had about 50 pictures, 30 of them I was editing with the gallery in mind and quickly realized that I can't crop them like I had done before, as they were all going to be printed to fit into standard sized frames. We were hoping to sell a few, 16x12 and A4 being the most popular. We couldn't afford to put them into odd sized frames and print sizes, so I cropped them to the standard aspect ratios(is there even one).
My point is though that the 20 or so other pictures that I had of my previous work, I had to redo, as a lot of them were all odd aspect ratios. And this really changed the look of most of the pictures.
I think after that I've decided that unless a picture definitely requires an odd crop size, I am best keeping the aspect ratio the same.
Hope that made sense.