I've done both methods. For a true non-wonky log cabin, I prefer to have pieces cut to the correct size first. That's a fantastic test of your scant 1/4" seam, and it's also self-correcting - you get instant feedback on your seam size with every single piece you add! I even made myself a box to hold the cut pieces for my most recent log cabin using plastic canvas and yarn. Worked like a charm.
Here are the blocks on my design wall, and a photo of my box for the pieces.
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I also made an intentionally wonky quilt from quarter log cabin blocks using the add first and trim after method. Very freeing to not worry at all about length or width of pieces!
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