Well your graphic design snobbery is welcome to come over and spot the differences on the layouts.

No but really, most professional printers print somewhere between 150 and 300 ppi so an 8x8 (2400x2400 px) at 12x12 will still give you 200 ppi which is plenty good enough for a high quality print. I've only done it a few times, mostly when I printed the same layout as both an 8x8 for my album and a 12x12 to frame and I fully expected there to be a difference and there wasn't at all. Persnickety Prints will actually let you print an 8x8 up to 18x18 before it warns you of it being low resolution but uh, I don't know that I'd recommend that.