I use heavy starch, especially in situation where there are bias edges. Along with careful pressing, the starch really seems to help prevent distortion. Also, If I am concerned about getting good intersections, I will first use my machine to tack/baste the areas that are problematic, open them up and check to see if they line up properly. If not, it's always easier to take out just a few stitches than an entire row. It takes longer, but I find that it works well for me. I have only done a few y seams so I can't help you there, except to mention the index card technique--since the lines on most index cards are a scant 1/4" apart, you can use those to help mark the stopping and starting points for your y seams--speeds things up a little over having to pull out a tape measure or ruler.
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