All l've ever worried about is when the fabric has a definite direction. If so, I make all the pieces go the same way. Otherwise, I usually do the fabric strip approach.

This is a great opportunity for a creative back design. Perhaps some orphan blocks for the strip or use a strip of coordinating fabric between the 2 butterfly pieces.

Check the selvage direction when piecing.If the selvage of piece #1 is on the left side, the selvage of piece #2 should also be on the left when you sew them. This keeps the print design going. Some prints won't matter if they get turned.

I pin a sticky note to the top of the piece to keep it in the correct position before sewing.

I made a Log Cabin for our King bed. Being that large I had to do some piecing. What I did was take leftover blocks and used them as a strip down the middle of the back. It worked well.
I did another quilt one time where I used a backing that I didn't really like and it bugged me every time I looked at it! Lesson learned.

I usually use fabrics of the front blocks I have made. I piece whatever fabrics I can to make up enough to cover the back. Sometimes I have extra blocks that I have made and I put those down the middle. I also have put a strip of fabric down the middle too.

I'm not fussy about backing seams. If I can match them, I do. What I am more fussy about is that the backing fabric is of a similar theme as the front... such as not using a juvenille or a feminine print on the back of a masculine top.

I have matched a fabric for only 2 quilts. They were pieces that were designed to compliment the fabrics on the quilt top. Most of the time it will not matter. It is always the quilters choice. Have fun with the butterflies.