I have done about a half dozen that way, but I hope to never do them again. If someone insists on it, I'd probably do it, but they'd have to pay extra.
They're a pain to get them to slope correctly without having a corner sticking up somewhere, and even then it's likely they won't slope like they really need to. If 12x12 was my only option, I'd cut them down to 3x3 or 4x4. Lots of extra work with the cutting and laying each piece individually, but less trouble with the slope.
I still have one shower pictured on my website (www.trevathantile.com, it's the second shower in the list of pics) with 12x12 on the floor. I had a heckuva time trying to get it to come out right. You'll notice that I turned the tile on a 45 degree angle which seemed to make it a little easier, but still a pain.
My advice: avoid it if at all possible.
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The top ten reasons to procrastinate:
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