The use of the phrase "no cap" is meant to convey authenticity and truth. The phrase originated in reference to decorative gold teeth, which can be divided into two distinct varieties: permanent gold teeth (aka "perms") or caps (aka "pullouts"). Whereas caps can be pulled out with ease, perms, as their name suggests, are permanent. They cannot be taken out for a job interview or court date. They are an honest and lasting expression of the owners' realness.
I would only get perms because people who wear pullouts are fake, no cap.