Pop-up flashes are designed to make people look ugly. Never use the pop-up flash on anyone that you don't hate. Pros don't get paid to make people look ugly. That's why pro bodies don't come with a pop-up flash.
Only use the pop-up flash if:
1) You want your subject to look ugly.
2) You are desperate and it is all you have and you would prefer an ugly picture rather over a blurry picture.
3) As a fill-in flash when you don't need much power.
4) You are trying to demonstrate to people how bad the light from a pop-up flash is.
Pop-up flashes are small. Small light sources create hard shadows. Hard shadows rarely look good on people.
Pop-up flashes are very close to your lens. Light coming from close to your lens is flat and unattractive. It also causes red-eye.
Pop-up flashes are weak. On the one hand, their weakness limits how much ugly light they can spray. On the other hand, it makes them lousy fill-in flashes, which is just about the only non-ugly use for them.
Pop-up flashes are great as controllers for external flashes (if your pop-up can do that). They are almost useless for anything else.
If you want better flash pictures, make your flash bigger (with a diffuser, softbox, or bouncing). Get your flash away from your lens (with a flash bracket, offshoe cord, or bouncing).
Be nice to your subject and your viewers. Don't use the pop-up flash.