I got the Thread Therapy DVD from Superior recently, and Dr. Bob does give that ironing caution about Bottom Line on the DVD. Incidentally, this DVD is free from the website, just pay about $4 for shipping, until December 11th. It's really worth it; lots of good information in it. Here is a link to the special offer:
https://www.superiorthreads.com/shop...hread-Seminar/
However, I think the best thing to do is to actually test with your iron to see for yourself if it melts or not. I bought a spool of Threadart 60wt polyester bobbin thread from Amazon just because it was really inexpensive and I was curious about it. I sewed some test seams and then held my Rowenta iron on the thread for more than 10 seconds, on its hottest setting, and there was absolutely no melting or other damage to the thread. I ***love*** this thread for piecing! Not only does the bobbin hold a lot more, so I run out of thread much less frequently, but it's easier for me to get accurate seams because it takes up so little space in the turn-of-the-cloth. Plus it's not linty, so I don't need to clean my machine as often! Here's a link to the spool on Amazon:
http://www.amazon.com/Bobbin-Thread-...dp/B0034W118Q/
I recently tried Connecting Threads 70wt poly thread in my midarm, because I really like the quilting thread to not show a lot, but it broke a few times. So now I've ordered a spool of Bottom Line. I'm hoping that being just a little stronger, it will work well in my midarm. I found that the least expensive place to order Bottom Line and King Tut is from this website:
http://www.kenquilt.com/