for backing fabric....
Print or solid..... depends on the top- often a print
seam down seam across.....across since i'm a long armer---always across---if not being long armed it does not matter so much.
cut off salvage edge ???? ALWAYS!!! the salvage is a tighter weave and will shrink differently than the rest of the fabric. the salvages should always be cut off --- then you can always save salvages & make something with them- or pass them on to people who use them for other projects.
match bobbin thread to backing color or..... of course
match bobbin thread to top thread so it doesn't show on top ????? if your tension is right your bobbin thread will not show on the top- the bobbin should match your back- the top thread should go with your top. even tension keeps both on their respective side.