No, the safe margins have nothing to do with it. If your clip is smaller or larger than your frame size you will see a black border or only part of the clip.
You can use “scale to frame size” to force the clip to the project (CS3)/sequence (CS4) size, but you may prefer to correct the initial settings or even the clip.
You will also find links to many free tutorials in the PremiereProPedia that will quickly show you how things are done in Premiere Pro.
Cheers
Eddie