I learned two very helpful hints for keeping 1/4" seams 1/4" from start to finish. Both were learned from Sally Collin's book "Mastering Precision Piecing" book which I highly recommend.
The first recommendation is to use a starter piece, it keeps the machine from sucking the thread down into the machine, and saves thread.
To start .... when starting to sew a piece, put your needle in the down position. Raise your presser foot just a smidge (not enough to disengage the tension discs, but enough to slide fabric under) and slide your fabric under the foot all the way until it just touches the needle, then position it to the right until it lines up with your 1/4" guide (whether a foot or a marked area on the machine). NOW you can start sewing and the start point is 1/4"!!
To finish ... use tools to help guide the fabric through all the way to end when your fingers can no longer hold the fabric. Sally uses a stiletto to help guide that last big of fabric, I use extra long pins that are in the fabric (from pinning it to hold the seam), and the pin heads are now sticking out way to the left of the presser foot. I always stop my machine when I get to the last 3/4" and re-position my hands, grab the pins or the stiletto, or whatever I am going to use, then continue sewing. IF you messed up and the fabric slid a little bit, no problem .. turn the fabric around and use the "start" method described above to re-sew the offending end, then rip out the boo-boo.
I am a big fan of sewing small pieces (finished size 1/4" square), and before I used Sally's method I fought with my starts/stops going west on me. With these tricks ... I have NO problems!!