I have always used cotton for piecing but would be willing to give poly thread a chance.

I just use whatever I have handy that is the right color range. Most often it is Coats & Clark all purpose. Sometimes it is whatever is in those little multicolor kits from the $ store (keep those handy for travel use). Occasionally it is the cones (either large or small) that I've picked up wherever I spot one in a color I don't have on hand. I'm sure at least some of this is all poly. I've never had a problem with thread melting, although I've had issues with certain scraps shrinking perceptibly when pressing. I'm guessing those scraps were acrylic or possibly polyester.

I use whatever I have.

JeanK, I'll bet that thread was nylon. I've bought nylon "invisable" a couple of times and hated it. It's wiry. And it melts. I've used all kinds of poly. Good ones are at least as soft as cotton and do not melt at any ironing temperature the fabric can take.

I use whatever I have. Ignore those who tell you you "have to" do/use anything.

It all depends on what you wish for your quilts in the future. Nylon thread will eat cotton. It is not open for debate. Now, do you care? Do you want your quilts to last for five years, ten years, a hundred years? Your call.
1.) Use like thread top and bottom matched to the fabric you use.
2.) Use like thread top and bottom, regardless to the fabric you use.
3.) Use whatever is cheapest.

One gives you most permanence over the long term. Three gives you the least. Second gives you a huge variance depending on use and cleaning of the quilt.

tim in san jose

i have used the invisible poly thread but the last time i used it is the last time i will use it the tread broke and jamed my machine i was forced to take it to a repair shop it cost me 60 dollars to get it fixed

I have had a poly thread melt under the iron as well. It was at a class so the iron was not getting a chance to cool down and maybe it was extra hot but that was the end of the piecing with poly for me.

i also use what I have on hand. And like others, i have lots of thread on hand from almost 50 yrs of sewing. The only thing that I am picky about is my hand quilting thread - YLI glazed cotton.

I use Gutterman Mara 100 for piecing. I buy the thread at Wawak.com when they run it on sale for
1.99 each. I tried it for FMQ but it causes the same problems I have with the Aurifil. It jumps out of the
bobbin case. I need to pick up some of the washers that they sell for the bobbin threads to see if that
helps. It only works if I am FMQ very slowly. Right now I am using the Mettler silk finish cotton for the
FMQ but that stuff is expensive.