If sewing to contribute a block for a quilt I sew scant 1/4" seam. If sewing for myself I see regular 1/4" seam.

The SCANT 1/4" seam
 

Over the years I have found the importance of a correct SCANT 1/4" seam, so here are my tips, for what they are worth.

If you have one always use the single hole sewing plate.
Remember the scant 1/4" should include the width of the thread and I do find since changing to Aurifil thread 50 I have much better seams that lay flatter.
A well maintained and clean sewing machine, especially the bobbin area
A good 1/4" patchwork foot, PLUS a seam guide is the "belts and braces" option for me :)

And remember a SCANT 1/4" seam it is only a measurement so if you can do a 3/8th seam or any other measurement you can do it, just remember what it is a 1/4" that includes the width of the thread.

And of course good needle frequently changed, never skimp on needles

If my machine starts stuffing fabric down into the needle slot, that tells me my needle is getting dull. Otherwise, I rarely had that issue, especially since I tend to chain piece and use leaders & enders.

For me, key was to make a note of the settings on the machine once I had my seam allowance dialed in so I don't have to figure it out again all over again each time I use the machine for piecing. On my 6600 I discovered I could change the default needle setting, so now when I turn my machine on it automatically moves the needle to the correct position to give me a good seam allowance with my quarter-inch foot. I have several machines that I switch between using; I usually have a post-it note floating around in the accessory tin with particulars on how to get the right seam allowance on that particular machine. The exception is my Rocketeer - if I put my quarter-inch foot on that machine, it has a precise scant quarter-inch seam without any adjustments at all. Like magic. :)

Also look to your 1/4" foot and single hole faceplate for guidance. On my Bernina my 1/4" foot is sized for a 1/4" seam width. The faceplate has guidance marks for lining up 1/4" seams. Plus, the plastic tray that fits over the base to give me a wider base to sew on also has markings to line up for 1/4" seams. Using all these I can sew half-square triangles with only one drawn line from corner to corner and sew accurate seams.

I'm really looking forward to getting either a KNK or Scan 'n' Cut for Christmas since either of those will mark my seam line on the fabric. I got a wonderful 1/4" piecing foot with edge guide & a narrow cutout that I found is ideal if I feed my fabric so the marked seam line is always visible through that little slit. I get that perfect 1/4" every time ... if only my cutting skills were as good as my sewing. :eek:

Sounds crazy I know, but thru much trial and error and trouble shooting with experts . . .turned out, my issue was thread. I now use only Aurifil thread as its approx the same size as the threads woven in my quilting cottons. Additionally, needles make holes each time they enter the fabric. The hole displaces threads, so take up room. Now, I use only Schmetz microtex 70. My quarter inch seams are now perfect.

I use the 1/4" foot on my Baby Lock and it works like a charm. The 1/4" foot with the guide is actually over-sized. Drove me nuts until I figured it out.

I do the three strip test every single time I start a different project and change my machine settings to accomodate any change in thread and fabric used. There's simply no way the settings for batiks and 50 wt thread are going to be the same as those for Kona solids and 40 wt thread. Seam allowance is a moving target; finished piece size is not.

That seems to summarize the quarter inch seam situation well.