Accurate block placement and placement of units within a block. If I accidentally sew a block into the quilt the wrong way or have accidentally inverted a unit within a block, like say there is a 4P unit in the block and I have sewn it into the quilt so that the dark unit in the patchwork is where the light unit should be, I will take it apart and resew it as long as the quilt is still a flimsy. Even if the misplaced block is in the center and will be a royal pain to fix, I will fix it. If it is already quilted I leave it but get rid of the quilt. Either donate or gift it to someone who doesn't care. My eye is drawn to the mistake every time and will zap all the joy out of the quilt for me (which is why I pass it on). For me, once it is seen it can't be unseen. But that is just for me and my quilts. Someone else's, I do try to bring it to their attention if I can and it isn't too late so they have the option of correcting it before it is quilted (just in case they feel the same way I do) but otherwise I say nothing.
Other rules for my own quilts and quilting:
Well constructed, lays flat and quilt is square. Being truly square (IOW my corners are at a 90 degree angle) is a big thing for me because I quilt using a LA and it is a royal pain to load an out of square quilt. Intersecting units line up to within 1/8" anything off more than that I take apart and redo because it will bug me. Same rule for chopped points if I have only chopped 1/8" it is good enough. More than that, I take apart and redo. Good tension in quilting, for my own quilts I can live with the occasional pokie on the back but not if the quilt is for show or for a client, then I rip and redo.
I always, without deviation, measure and cut my sashings and borders to size before sewing them on. This helps to ensure a nice square quilt that will lay flat. If I have mitered borders, I will measure and cut but give myself a little wiggle room for the miter. (usually an extra inch or two). But I still measure and mark and pin for the inside part that will be sewn to the quilt body.
If the quilt is for a show I up my game considerably. I won't allow any chopped points or intersections that aren't spot on.
I prefer to work with 100% cotton fabric when piecing. That is from experience of working with other things (blends and synthetics) and not enjoying how the blend or synthetic behaved so I didn't enjoy working with that fabric. I do like wool applique and probably wouldn't hesitate to mix wool with cotton in a quilt. I also won't hesitate to mix different kinds of cottons, like batik and a print or even mix in a flannel with regular cotton or a stablized cotton T-shirt knit with regular quilting cotton or heavy denim. I haven't yet used silk or the silk cotton blend "Radiance" but I think I would like to work with that as well. I know it shows off quilting beautifully.
I do like to use lots of different threads, cotton, poly, silk and all different weights. It is amazing how different threads and different weights of threads can be used in quilting and embellishing and can have so much impact on the finished piece. For piecing I stick to fine thread to improve my accuracy. Again I won't hesitate to mix cotton and poly threads in piecing. My standard combo is aurifil cotton on top and Bottom Line polyester in the bobbin. Unless I find something I like better, or one of these products gets discontinued, I won't deviate from it.
I am a rebel and iron more than I press! But I don't use steam and I only spritz with a spray bottle when I am ironing yardage so have not distorted anything by ironing (as opposed to pressing).
I'm sure I have many other self imposed quilting rules but these are the ones that come to mind.