I had heard once that stitching in the ditch was not good for the seams, but that may before good machine stitching (seams, that is).
Personally, I like to see the quilting, especially if I did it by hand, so in the ditch can be less visible.
1/4 inch from the seam is easy to do: after a while you really know what 1/4 inch is!
I have done diagonal lines and such and find my stitches start to wander, even if I think I know where I'm going...
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