If I don't wear a retainer after braces, will my teeth go exactly back to how they were? I have not been good about remembering to wear my retainer, and am beginning to wonder if I really even need to..

Investment. You invested a great deal of time and money resolving your malocclusion. I can guarantee you that if you do not wear your retainers as prescribed, your teeth will shift, slowly at first, then with increasing rapidity. Don't seek absolution, seek compliance, or you will be greatly disappointed.
The . The only way you can be sure that your teeth will stay straight after orthodontic treatment is to wear some type of retainer. If you are not good at remembering you might benefit from a fixed retainer that is bonded to your teeth. There are some people who's teeth stay put after orthodontic treatment, but that is rare. Do you feel lucky?
They . They may not go exactly back to where they were but they may end up where you don't want them to be. If your orthodontist told you to wear it, then it's necessary. After all of the time, money and effort that you put into having braces you should protect your investment by wearing your retainer.