Is there a reason to do this?
 

I've seen people pre-press their seams before pressing them to the dark fabric or whatever way they press them.

Is there a reason for pre-pressing? I do it because I've seen people doing it on quilting shows but I really don't know why.

It's supposed to "set your seam." No, I don't do it.

Have you ever pressed a seam the wrong way, then had to press it back the other way? It seems like the same thing to me and I don't want that effect. And if you're pressing directly on a seam with polyester thread, on "cotton" setting, that could melt the thread.

I've seen/heard that done before also. I've tried it both ways (pre-press & just press one time). I really can't tell that much difference but they say it's to set the seams (whatever that means). So that being said...sometimes I pre-press & sometimes I don't...just depends on how I feel at the time. :)

Does it press the thread into the fabric? As the opening out of the seam takes a little of the fabric in the 'fold', (hence the use of a scant 1/4") would this make the seam allowance more accurate when you press the block open?

Or, am I overthinking this? :D

Setting the seam by “pressing” it (up & down, not “ironing” back & forth) might be a carry-over-thing from garment making (e.g. sewing a fabric with a lining). You want a balanced seam, no puckers, smooth/flat/even together, before you grade, all that, so setting the seam comes first before pressing-ironing it open, or to the side. Or, you might need a perfect scant seam allowance if doing a specialty seam, or an edge finish. I don’t normally do it when piecing a cotton quilt. But I might, with a curved seam, or if I graded something.

My understanding is that it is as Bobbinalong said.....to settle the thread into the fabric. I don't know whether it is strictly necessary or not but I always do it...mostly out of habit because I was taught to do it.

When I first started quilting many years ago, I did not do the pre-press and often had a fold of fabric at my seam line. Then was watching a quilting program on TV in the 1980's and she advised pressing the seam before pressing the seam open or to one side. I tried it and found I no longer had that little fabric fold at the seam so I have been doing it every since. Just something that works for me.

I've also seen them do it on the shows but when it comes to doing it I forget! :DI think it
supposedly flattens the thread. Who knows?

MSQC Jenny does it on her toutorials. I tried it and it didn’t change anything!

Thanks for asking the question nanna-up-north. I've been wondering the same thing. I never set my seams before when sewing (garments and home dec.) and since I learned it for quilting I do it all the time.