OdessaQuilts has Very good guidelines which I totally agree with.
One think I haven't seen addressed is if you are measuring your cut from your cutting mat or your ruler. I find that if I measure from my ruler I have less problems. I also put a strip of tape, like 2 layers of the blue masking tape on the line where I need to stop, if I'm cutting 2" strips I put it on the BACK (down) side of my ruler at 2", leaving 2" between the edge of the ruler and the tape, so that the tape stops the ruler from sliding on the fabric at exactly the same place each time I lay it against the edge of the fabric. I usually cut 4 layers at a time. I hope you see what I'm saying?
You can also use the layered tape on your sewing machine as a stop at 1/4" to guide you fabric at exactly the same width on each piece, which helps a LOT. I use a 1/4" foot with a 'bumper' on the side to help keep my seams all the same. I also use the same machine all the way thru a project for consistency thruout that project.
My last suggestion is that only God is absolutely perfect? A lot of us are perfectionists but I have come to the conclusion that life is too short and I have too many quilts to make to worry about everything being perfect. When it's all one big quilt and the quilting is done even I can't find some of the mistakes that I thought were glaring at me when I did the individual block.
Good Luck!