Cropping - I'm all for it if needed, and sometimes it's needed. For some reason I'm more inclined to crop 35mm to a square or whatever proportion looks good, but with 6X6 I like to keep it square if at all possible, and use the whole negative. Don't know why. :-)
To the OP - the 124G is a great camera - not a toy, but not a Rollei either. If you have some money and want to have some fun then sell the Mat and get a recently CLA'd Rollei. It's still a TLR and the quality is better after all. Your images might improve when you use a better camera, but then again maybe not. And who cares. Just have fun; that's what it's all about with film.
I have a Yashica TLR which I like. Then I got a Hassy and two lenses thinking I would enjoy the better quality glass, the ability to change the lenses, the film backs, etc. Turns out I don't give a hoot about most of that stuff, and I hate the noise and the mirror slapping around in there, so now I'm shooting the Yashica again, and appreciating it for what it is: simple, small, silent, non-vibrating. It really is a nice camera.