A lot depends on the reason I am making the quilt. If it is for a gift, I am careful but don't sweat too much because usually the recipient won't notice the mistakes anyway. As long as it presents itself well and is put together well, a few minor boo-boos don't bother me. If I'm making something to sell, I'm pickier, but again, the end buyer probably won't even recognize most of what I call mistakes. Now if I'm trying to learn a new technique, then I'm really picky because the whole point is to master what I'm learning.
I tend to be the hardest on myself when I'm making a quilt for ME because I know will see all of my mistakes every time I look at it. I know what will bother me, so I fix it. And there's something about finally getting it right that just really makes me happy. But I do it for me, not for anyone else, including the quilt police! And sometimes I never do get it quite exactly right. Then I just move on, maybe come back another day and try again, or maybe not. I don't let it get my knickers in a twist.