I've also felt that part of it is when you entered the hobby as well. Having only bought pre-war cards for the last two and a half years, I never felt any stigma associated with buying knowingly trimmed cards, or even gave it much thought, other than a great way to buy sharp cards at a tremendous discount. I think if I became involved earlier or was more interested in high-grade cards than low-grade cards with an affinity for period writing, perhaps in my youth, I would feel differently about the practice of trimming as well.
From my perspective, I paid the same amount for both Mathewsons shown below and was very happy with both of them, though the back on the Dark Cap is mostly covered in scrapbook paper and the lower right hand corner on the white cap was trimmed to make five, sharp corners.
I have number of other knowingly trimmed cards and never felt they detracted from the aesthetic of my cards or collection.