Two of my most admired photographers, Michael Kenna and Bruce Percy, photograph in 1x1 aspect ratio, exclusively (no "L" bracket required). They are masterful image makers and the square format is only a small part of their signature styles, but it is their vantage point in seeing the world.
Until recently, I had always felt that if I shot in full-frame 3x2, having the final crop in mind as I made the image, there was no need to change the aspect ratio in camera, nor potentially "sacrifice" those coveted megapixels we all consider when purchasing gear. After all, whether you crop in camera or in process, the net result is identical, right? Mathematically, yes, artistically, no.
When I posed my "shoot in 3x2 with crop in mind" approach to Bruce Percy, he responded that that is not enough. His views are that 3x2 is too stretched for portrait, and generally so for landscape. It is far easier to work in 4x5 and that your images are tighter and cleaner. Further, and this is the key, if you set your aspect ratio to 4x5, or 1x1, your perception begins to shift, and quickly. Initially, I resisted, but after grudging experimentation, I realized that he is absolutely right. As I worked in the new aspect ratio, I also found that it affected focal length choices in numerous ways, and it is a fascinating process. Composition has always been my utmost concern, and working in new ratios opens your eyes to new configurations and hard decisions about what should be in the frame. These are all good things. As Guy Tal wrote in his exquisite new book "Another Day Not Wasted", "It matters less what is in the frame, than why it is in the frame".
As with all new skills, this can be a struggle at times, with the usual steeper-than-anticipated learning curve. But, why else do we photograph, if not to grow creatively and enjoy new worlds, especially those that are already in front of us.
Now, I change the aspect ratio out of 4x5 only if the subject absolutely demands it. Clearly, no one aspect ratio will work in all situations, but I suggest you test this out. It will open yours eyes in the simplest and most dramatic way.