How much does it bother you? If you are of the free-wheeling temperament that is rarely concerned by little problems, then you may just decide to keep on working. But, if it bothers you enough to stop and consider redoing it, then chances are it will bother you later. In the balance of things, I find that the time it takes to re-do a small area is insignificant in comparison to the time that I will have to look at a piece that I know is not right.
Can you do better? If the mistake is made because the technique is one that you are just learning, then chances are that doing it over may not produce noticeably better results. Since I regularly like to try new approaches or styles of work, I am always a “beginner” at something and my technique is not perfect. When things go really wrong I take them out and do them over, but if the problems are minor I tend to leave them. Small problems left in place can be considered just another milestone as we measure our progress.
How important is it? If the piece is not intended to be an heirloom, will never be judged, or will be given to a non-quilter who will not notice, then leaving minor problems may be the right choice. Getting it finished may be the focus. If the quilt is meant to be a show or competition piece, then it is important to make sure that every step along the way is the best that we can do. I have an important piece in which I took out hand stipple quilting because it was not giving me the texture that I wanted. Not an easy task! But I am much happier with the change that the new quilting made. It was the right decision. And, in a most extreme case, my advice to a good friend who was not happy with a quilt that was three-quarters quilted was to take it out and do it over. She had spent a long time in creating an original applique quilt that she intended to show on the national level. The quilting that was already in the piece had problems both technically and visually, making the quilt less than it could be. It took my friend a while to make such a big decision, but in the end, she did take the quilt apart. The new quilting she added is spectacular in both design and workmanship and the quilt won several major awards.
In the end it comes down to your own temperament, where you are in your quilting journey, and where the quilt is going. Satisfaction in our work comes in many different ways. Here’s hoping that we all make quilts that make us happy.