Another question from me, I've been asking a lot here lately. But you all know so much :-D

Anyway, the actual question is about the thread type I'm using to hand-piece a quilt top. I've been using 100 % cotton thread and have pieced about half my blocks with that. Unfortunately, I'm about to run out of thread. I do, however, have some polyester (or maybe it's a mix of polyester and cotton, I'm not entirely sure) thread I could use. Will this matter when I piece all the blocks together? Like make the top 'pull' in some places or otherwise look 'off'? Or can I just switch without consequences?

If the thread shrinks at different rates when washed, there may be a difference, but probably not too much.

I don't think there will be a problem.

Is there a way to test that? I know you can test bleeding and shrinking of fabrics by putting a bit of the fabric in hot water and letting it soak for awhile, then letting it dry. Could you test the shrinking of thread in the same way?

I don't think so, but go ahead an go for it. I've mixed cotton an metallic thread in the same quilt without any problems.

Thanks! I'm just going to try and see :)

i don';t know any reason why it would give you a problem. I do it all the time.

i've been sewing/quilting for many years now...i've used what ever thread is right there...over and over, if the color is ok...and it is not breaking constantly i use it, i do not pay any attention to what the last spool i emptied was i just start again with the next one...i've never had a problem with different threads in a project...i've only had problems with some threads breaking alot in my machine and problems with invisible threads...which i hate using...beyond that i don't (sweat the small stuff)
i do tend to do my hand applique and piecing with silk thread but that is just a personal choice, i just love hand work with silk thread, it is so smooth and easy to use....but if i run out and have to use cotton, or rayon or polyester i don't worry about it....too many other things in life to worry about -- ;)

I'll admit that I am not on the 100% cotton bandwagon. I just use whatever I have on hand regardless of content and have not had a problem.

Well, I'm the oddball here as I would follow through with what I was using on a particular quilt. I would be concerned that there would be a difference in the long term with the different threads on a much loved/used washed quilt. However that said, obviously from the comments already made people do it all the time without any problems.
It looks to be more personal preferences and what people are comfortable using....