I'm not sure what you mean by this - are you saying sharp corners can be made at any time while still maintaining correct card size and while also being undetectable to the major grading companies?

If an oversized card is trimmed to its correct/intended size, I don't care (it was done all the time in the 1970s and 1980s, and back then we actually thought of it as a good thing); if a card is correctly and professionally trimmed from a sheet or strip years after its date of issue, I don't care.

If a card is "stretched" and then trimmed, I absolutely don't approve of that - I'd consider that altering/doctoring, because material is being subtracted from the original card. But if it's undetectable, I don't know how you'd combat it. Avoiding cards with sharp corners may reduce your risk, but I don't think there's any shortage of manipulated cards in low grade collections, either.