I had my braces removed 15 years ago. i have not worn a retainer and my teeth are perfectly straight. should i get a permanent retainer for bottom teeth and removable for top to prevent movement now?

Yes on retention: Even though your teeth are still straight, almost everyone's teeth move to the midline with age. They most likely will shift in the future. I personally don't like permanent retainers because it makes cleaning those bottom front teeth difficult. Removable retainers for both upper and lower would be perfect and just wearing them once a week at night should keep those perfect teeth straight!

Shifting: Shifting will occur throughout your lifetime. You are one of the very few people who dodged the bullet and did not have relapse. Have your dentist check your occlusion for any signs of instability. If there are any, retainers may be a wise choice.

Retainers: You are one of the fortunate people whose teeth are stable after braces, but as we age the teeth can shift. Wearing a retainer a few nights a week can prevent this. A bonded lower retainer would make it much harder for you to floss and keep your teeth healthy so i would not recommend that.