I really try to not piece batting but of course I have! I think that serpentine idea would be awesome on like a warm and natural type batting. Otherwise, If I can I like to use the outer edges for nice smooth edges, or just cut nice edges, butt-join and whip stitch together. Once you've made the batting the size you need, you should be able to just sort of forget that it was joined.
For my fluffy poly batts I do a tiny bit of an overlay, smashing the fibers around so they are smooth(er??), and simply whip together with a huge hand zig-zag.
Just in general, you do want to avoid seams in backings or battings down the middle/normal fold lines. Just a couple inches one way or the other is enough. So think in thirds, whether horizontal or vertical, if your yardage is wide enough that two widths will do you, put a whole width in the middle. Slice the other piece down the length and put one side on either side of the whole piece. While the temptation is great to use the selvedges attached and some fabrics are good for this, for the most part, it is best to remove them. They are thicker which can be felt or impact your quilting, and can store tension (make ripples and such) in the fabric that is released by cutting them off. I advise open 1/2" seams for backs but I've used 1/4" too.