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If I come back to this again and again when explaining sashiko stitching it is because it worries me! If you don't leave some slack thread on the back of your sashiko stitching the fabric will pucker, especially after a washing or two.
Study this photo a bit. You can see I have left the thread quite loose where I have carried the thread from one line of stitching to another.
But it is also important to leave little loops of thread in the stitching lines themselves. I suggest about a quarter inch every six inches as a rule of thumb. Another good rule of thumb is to leave about a quarter inch loop whenever you turn a corner.
If you are confused by the 'white fabric' I'm stitching on, you can see what it is and why its there here (and no, you don't need it :-)
The more dense the sashiko design you are stitching, the more important this is. For example, this design needs a lot of looseness:
This dense sashiko stitching style is called hitomezashi. You can lean more about that here.
This design won't need as much looseness in the threads, but it still needs some.
Two factors cause the fabric to tighten up as your stitching progresses. One is just the fabric grain being tightened as more of the thick sashiko thread is stitched into it. The other is that sashiko thread is 100% cotton and will shrink a bit when it is washed. This is a good thing, it makes it plumper and prettier. Presumable your fabric is a natural fiber and will also shrink a little bit. This is what creates that beautiful 'quilt-y' look, but it is not a happy effect if it over tightens!
Can you leave too much slack thread on the back? Not Really. Sashiko thread tends to stay where it's put :-) It isn't likely to loosen up on the front after it has been washed once (or even before). And if that did happen you could simply pull it back onto the back of the fabric. I have only ever had it happen if I snagged the stitching on something - and it was easy enough to pull back to the back.
What to do if you didn't remember to leave enough slack and your fabric is puckered?Cut a thread or two on the back and ease the stitching /fabric flat, then restitch the area where the thread pulled out.