I started out with printed backings to match the fronts as I've only made large lap quilts (some are actually twin size, but will be used as lap quilts.) Then I came up short of fabric and used some braids to make it bigger. Now I like stacked coins or something to break up the expanse of sameness. I have even made a full quilt for the back because I didn't like the front - now I do, so that daughter got a bonus. I use thread that may match something in both sides. My FMQ machine (Singer 301) doesn't show anything so I can have red varigated on the top and yellow on the back. I did like a blue varigated on the back of a dotted dark blue back. It was a QOV and the quilted stars showed up that way. In others words, don't worry too much. Once it's stitched, it all goes together (within reason, of course).

I've even put pictures on the back when I ran out of space on the front. I was concerned about the quilting, but finally decided the front was the feature and quilted like I needed to, though I did avoid the girls' eyes.

I usually use a print on the back. Sometimes is in the quilt top and sometimes not. for thread I usually use something that blends. If there is enough of the fabric left from the top to make a scrappy back I will do that.

I usually go for prints on the back...something to coordinate but not match the front.
Usually cut off selvedge but not always
Seams on the back - wherever they need to be to make the backing fit the front
I hand quilt so there is no bobbin!

I use prints and solids for backs have also pieced the backes. Salvage off, I long arm so back seam across or any which way when I piece the back. Thread- that depends, usually nutral or a color that blends. Latley I have been trying contrasting colors, boy do I have to be careful with my patterns!LOL

Prints on the back where possible. Take off the selvages. I seam on the diagonal (ala John Flynn) for lap quilts and piece the larger ones.

Most of my quilts have solid or ToT neutral color backs with a contrasting thread so the quilting really pops on the back. I've done a couple of pieced backs - mostly to use up small amounts of fabric i had left over from the top. I have a couple of quilts that have a white back that I had white thread put on the back for a special effect. Sometimes the top has a different color thread than the back. Sometimes I have the top thread matched to the top and even to the blocks so it seems sucked into the top. Sometimes Not. Seams? Whichever way they fit :-) All of the above are decided based upon the look and feel of the quilt top once it starts coming together. and selvages are always trimmed off. All of my quilts are LAQ'd.

As a longarm quilter, I always advise my clients to trim the selvage off then use a 1/2 inch seam. The selvage can tend to draw up. If you can test out your thread on scraps of fabric and batting you will be able to tell if your tension is good enough to use different color thread in the top and bobbin. If your tension is really nice you can use a plainer back to show off the quilting.

I made a scrappy quilt I call "Bull's Eye on Point". It had many different colors in it. I couldn't spend any money on a backing so I took 12" squares of many different materials that were in the front and made a back for it. I think it turned out ok. The only picture I have of the back is what Charisma posted. Here is the link.
Twinkie's Scrappy Drunkards Path...quilted by Charisma

when quilting I once heard a comment made by a judge when she was trying to find fault with a quilt nothing wrong on front so she was looking on the back to see how even those stitches were and the fabric was very busy really could not see any stitching and the judge said "One Smart Lady Did This" the quilt won a ribbon

I really don't have any hard set rules regarding the backside of a quilt.....but....I do have a few preferences....LOL.

1. I prefer not to use stripes or an obvious directional print (difficult for me to keep straight)..... small prints or solids are ok.

2. Pieced backings are ok and since I quilt all mine on my Janome 6600, the direction of the seams don't matter.

3. Bobbin thread color is individual to each quilt. Some quilts will just scream at me to use a thread color that will stand out. Others need to blend.

4. Selvages always cut off, as they might shrink differently than the rest of the fabric.

5. Always prewash....I take no chances with bleeders.