Do blocks need uniform thickness?
 

I started a unique quilt several months ago (my first one ever) and was so appreciative of the suggestions I received here. I have finished my 42 blocks and am ready to start putting sashes and cornerstones between the pieces. All of the blocks have been made from vintage needlework done by my mother-in-law (embroidery, crochet, applique on dresser scarves, tablecloths, napkins, pillowcases, etc.). I have backed all of the pieces with thin stabilizer because of the loose weave of the background material and the need to repair some of the embroidery. Depending on the original fabric, the blocks are uniform in thickness. Should I add another thin layer of something to the thinner blocks so that all are approximately the same thickness? Thanks for any advice you may have.

I would personally try to make them as uniform as possible. That will provide some uniformity for the whole quilt and keep it from needing much squaring up at the end of the project.

I'd also suggest using a wool batting for it--the wool has nice loft, is lightweight and will "even" up the weight of the blocks. Please post a picture when you finish--sounds amazing!

TexasMary ... You have me all excited to see your project come together.
I love it when we can combine our different interests into one project.
Yours is even more special with your MILs work being highlighted in your creativity.
I'm not so sure you need to equalize the thicknesses of each block.
Instead, let your batting do the balancing for you.

I do mostly scrap quilting and often have a number of different thicknesses of fabric. Usually not a problem, sometimes the lighter tight weave of a batik actually holds up better than the thicker traditional calico it is next to, but they aren't causing any wear on each other.

You've already fused the blocks with interfacing that should be enough unless it is very thin, I would probably not worry about it. If you can still clearly see through it then yes, I'd probably use a very light weight muslin or a poly broadcloth bright white.