Nothing smaller then a FQ goes into the washer loose. But...I wash even FQ's in a mesh lingerie zipper bag so they don't get tormented in the washer too much. I pre-wash everything - if it's a small piece I hand wash it. I saw the most STUNNING quilt at a show once (looked like it easily took a year to make with all the fantastic hand applique') and there were multiple tiny areas of color bleeding off (reds and burgundy) into the cream background. I bet that quilter wanted to HANG herself. That was all I needed to see.. I am a fabric washin' kinda gal.
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I wash everything, including scraps that are given to me, but I never buy pre-cut fabrics such as jelly rolls or layer cakes. I would not wash those if I had them. If you mix fabrics that are washed and unwashed, you probably won't notice any problems unless some of the unwashed fabrics bleed (and you would have that problem even if there were no washed fabrics in the group) or unless you are using flannel or homespun, which shrink a lot more during washing.

DITTO

I used a jellie roll, not washed, with pre washed fabric, the quilt has ben washed several times and no problems. I did hold my breath the first time I washed it LOL

I don't prewash jelly roll or charms, but I do wash and dry fat quarters. One quilt store owner told me that it's fine to mix prewashed with unwashed, because there isn't as much shrinkage in fabrics as there once was (thus the switch from 45" wide to 42"). I've never had a problem combining the two! Good luck!

I'm not sure if that's the exact reason for the switch from 45" to 42" but there are fabrics that I would not mix because they shrink a lot more than others. JMHO.

Thanks a lot to all of you for answers! I got the idea now

I just did a jelly roll quilt that was mixed with fat quarters. The fat quarters were washed, the jelly roll was not. However, as soon as I was done with the top, I put it in the washer, covered it with water, soaked it for a while, spin dried. Then into the dryer until almost done. It was kind of a pain to then press, but it came out just fine.

I have washed 1/4 yd (not FQ) cuts. I wouldn't wash anything smaller than that. But I typically don't use pre-cuts, other than fat quarters.

I just bought my first 2 1/2" strips for a pattern which calls for 27 unique strips. Since my (small but growing) stash is all pre-washed, I purchased 1/4 yd cuts for the extra pieces needed to have the full complement of unique strips. I won't wash any of it until the top is completed.

I agree - don't mix washed and unwashed.