How much material do you take to your first five-minute spot?

I’m realising “write five minutes” and “have exactly five minutes of words” probably aren’t the same thing once laughs, pauses and nerves get involved. For a first proper five-minute spot, would you rehearse something that lands comfortably under time and leave space, or prepare extra lines in case you race through it? I’d rather finish cleanly than discover my closer lives somewhere after the light.