I have a Bernina 1230, which is now a vintage machine (15+ years old).
The 1/4" seam for me is totally dependent on how I set up the machine. I ***must*** have a physical barrier for the right edge of my fabric. Trying to eye it against a line on the machine bed or against a foot simply does not work well for me; the only way I can get consistent seams is by creating a physical barrier. I used to use moleskin, but now I really like this Dritz product:
http://www.amazon.com/gp/product/B0085L2Y2I/
except it may have been discontinued. I like it because it is a little taller and firmer than moleskin. Dritz sells these adhesive strips in combination with a red ruler. Do ***not*** use the ruler to create your 1/4"! I measured it and it is a true 1/4" instead of a scant, so your resulting seam will be too big.
Because of the way my Bernina is constructed, I need to move the needle one position to the right so that I am not trying to put adhesive on top of the feeddogs. Here's what I do next:
I place my favorite ruler with thin lines (or you can use graph paper with 4 squares to the inch) under the presser foot of my machine. I lower the needle so it is just a little to the right of the 1/4" line. Then I lower the presser foot to hold the ruler in place. I check to make sure the ruler is running straight from front-to-back. Then I take a strip of moleskin (or the Dritz adhesive strip), remove the paper backing, and place it against the ruler's edge.
That creates a barrier of a scant 1/4" from my needle.
The other thing I do is use a fine thread in needle and bobbin. In my case that's usually Aurifil 50-wt 2-ply top and bobbin; sometimes I will use 60wt polyester in the bobbin. Many people do not realize that the size of their thread influences how their 1/4" seams turn out. This is because turn-of-the-cloth when you iron takes up some of the seam allowance. Using a finer thread allows a flatter seam with less fabric taken up in the turn-of-the-cloth.
I have done this so many times I no longer need to check it, but before sewing a quilt you should check the seam allowance. Standard method is to sew 3 2.5" strips together and iron. The measurement across these strips should be exactly 6.5" after pressing. If the measurement is less, your seam allowance is too big and you need to move the adhesive barrier closer to your needle (or move your needle more to the right if you have a machine with lots of needle positions). If the measurement is more than 6.5", your barrier is too close to the needle.
This is the ***only*** method that gives me consistently accurate 1/4" seams. If you haven't done it this way, I would suggest trying it before investing in a new machine.