Welcome to the board. Do you have some scraps left of your quilt? If so, audition different threads and see which one you like best. Just sew like you are quilting a with several different threads and go with the best for what you want your quilt to say. Sometimes we want them to whisper "I'm beautiful" and other times we want them to shout "HEY LOOK AT ME, I'M FABULOUS"
As far as your corners being perfect, no one gets them right all the time. (if someone says they do, stay away from them because they'll lie about other things too) LOL Just work on your cutting and it will get better.