I am making autumn placemats now
. I have made a number of sets over the last 7-8 years, and they get used (and washed) all the time. I have always used a single layer of warm and natural in them, quilted fairly closely (2" max). I didn't prewash the fabrics in most of them, and nothing dreadful has happened. They crinkle up a little, but we haven't had any problems with glasses or plates sitting on them. The only issue is to make sure you cut them big enough if not prewashing. My first set ended up smaller than I wanted.
Note that I have never considered placemats or table runners to be hot plates or trivets, so I don't worry about them taking a lot of heat.
Pam