bkay

I have. It was on one of my first quilts. Getting everything to line up is difficult. In my case, it was on an I Spy quilt, using blocks I had received in a swap. They weren't all the same size, so it was a really difficult job. I mean row 5 was longer than row 6, so after I had already attached the sashing to row 5, it wouldn't fit row 6 and line up correctly. I finally replaced a couple of blocks on row 6 to make it fit right.

Now, not only do I check that the charms are the right size, I sash each block. It's really easier than the long strip of sashing, even though it doesn't seem so.

If it's too late to change anything, make sure your rows fit together before you sew the sashing on. Then try not to stretch anything as you go. If you can cut your sashing on the length of the grain, rather than across the grain, the sashing will not stretch much, if any. However, if the rows aren't exactly the same size using the width of the fabric will allow you to ease it a bit.

Something I learned from a video: If one row (or block) is slightly longer than the other, put it on the bottom when you sew. It will help take up the slack.

Of course, we're all speculating a solution for you. We don't know exactly what you are trying to do as we have not seen a photo. You say you want to sew blocks on a long row, then you say you are sewing the blocks together in the corners, which seem to conflict to me.