Hi Korry. There's nothing wrong with checking your form in the mirror. Just don't obsess and stare of course. Another good way to check your form is to have an experienced lifter watch your sets. YMMV because if you ask a powerlifter on Monday, a bodybuilder on Wednesday, and a 117 pound girl on Friday to watch your sets of squats or deadlifts you will get 3 different opinions. Even then, each person is at a different experience level.

And another problem is that each lifter knows what works for themselves, not you. Many beginners make the mistake of projecting their own experiences onto everyone in the gym. I like to call it the know-it-all stage. It's the one just after beginner. So there are my reasons why I like your answer..."Hey, it works for me".

Good Luck