Hi Graeme, I'm pretty sure you've read some of my other comments where I've made it clear I'm not an HCB perfectionist, willing to bypass a good picture because it doesn't happen to fit the Leica's 3 to 2 aspect ratio, imposed on Oskar Barnack because the camera he designed used movie film.
But there are other formats. Nowadays I prefer the four-thirds format I have in my Pen-F, but during the sixties and seventies I spent enough time on the street with my Leicas that I began to see in B&W and a 3/2 aspect ratio. The time to crop is when you're looking through the viewfinder, but it's okay to decide during composition that you're going to crop when the format of your camera doesn't fit what you're after. On the other hand, the decision to crop should be an exceptional case.
Since your viewfinder doesn't cover 100% and you're composing (cropping) in the viewfinder I'd expect you to remove a bit around the edges -- the part that falls outside the viewfinder. Camera raw makes excellent lens corrections by the way, so I wouldn't crop because of that.
Bottom line: you're right. It's not good to bang away and hope to find something worthwhile on your computer monitor with a crop. But an awful lot of people do exactly that.