Old wives tale now. Maybe once upon a time it was true but with changes in manufacturing processes and and thread makers wanting to please quilters, the times they are a changing! I encourage you to visit Superior Threads education site. Loads of info there and also some myth debunking. Here is the old "poly thread will cut through cotton fabric" myth explained

https://www.superiorthreads.com/educ...ar-the-fabric/

Edited to add, everything I have learned about thread I learned from other members here linking to different resources. Then going to shows and speaking with thread manufacturers I have learned so much. Now my thread addiction is almost as bad as my fabric addiction. So many choices and so many things we can do with the thread once the piecing is done. Fancy metallics, heavy embroidery, cotton, silk, synthetics our choices are endless and we should take advantage of that.

Good to know feline fanatic! I have a rainbow of colors of poly threads for embroidering and sometimes if I don't have the right color for something I'm quilting I'll be like "oh that's the perfect color if only it were cotton." May have to dip my toes in these new waters.

Going back to the OP, I have bought tons of prewound poly bobbins off ebay for my embroidery and have yet to be disappointed. Having said that, I bought only black and whites in bulk .

the prewound bobbins have much more thread then I can wind on myself.

This is for piecing. I use cone of gray for the top and when I have the prewound bobbins in gray I can sew and sew and sew before I run out of bobbin.

I have not tried prewound bobbins ... I usually wind 3 or 4 at a time for the project I'm working on.

Any idea how many yards of 50wt thread a typical Bernina DSM bobbin holds? The Bottom line bobbins have 118 yards per the Superior website....

First - the only dumb question is the one that is never asked! Then, I think the only time the thread (or fabric) is an issue is if you were submitting your quilt to a super fancy quilting judge! I believe that whatever you choose to use to make your quilt is ok. Maybe the fabrics are different, maybe the thread is different, maybe the colors aren't everyone's cup of tea - but it is your choice. There is nothing harder and more of a drudge than working on something you don't like.

There is a difference over time, but that doesn't kick in until the cotton fibers start to break down at around 20 years. For the first 15 years, other than possibly a difference in the rate the color fades, it won't make any difference. After that, the poly fibers, which are stronger, will rub/pull on the cotton fibers and cause them to break more quickly than if it were all cotton. The other issue is that poly thread fades much slower than cotton so after a good number of years, threads that started the same color will be 2 different shades of that color.

I use pre-wound Janome bobbins on my Janomes -- the thread is a poly bobbin thread and lasts a very long time. If I don't want my quilt to draw up, I use poly on top and bottom when I quilt. Actually, I grab anything that is the color I want be it cotton or poly.

I think, for the purposes of the original poster's question, the real issue is not whether you use cotton or poly, but is more about the weight of the thread. You can get a lot more 60 wt thread on a bobbin vs a 40 wt thread.

I have been sewing and quilting for a very long time. I don't worry about cotton, poly or anything else. If it stitches I will use it.
Use whatever works for you.