What I do – for digital, I generally "shoot to 5x7", because I print to 5x7 as a standard minimum size for a lot of my bundle work.
So that means I “see through the viewfinder the scene with a sliver cut off the L&R side edges when holding the DSLR in landscape format.”
On my medium format cameras' screens: I have framing grids for 5x4 and 5x7.
You can get framing screens for some DSLRs or mark the interchangeable screen yourself, or just generally shoot a bit wide and allow for various crops later.
I’ve shot a whole sessions, using DSLRs to: square format; and others to Widescreen Format (16:9) – it is just a matter of practice of visualizing the final framing.
WW