I usually trade in 3-minute bits, and confidence, or the illusion thereof, is pretty important. In my case, my sights are set on a bunch of laughs throughout, but I'd would think especially if you're building one long slow burn into a huge punchline at the end, you can't waver in what you're doing.
An example: When the smurfs movie came out last year, I saw someone go up with 5 minutes of smurf jokes, which I'm pretty sure are just phrases with the word smurf inserted. It was... not good. But I don't think that guy really lost until he bailed out and spent the rest of his set flipping through a notebook looking for things to say. Any one of those jokes was popsicle-stick quality at best, but the sheer fact that he was still going would have brought the house down by the end of it, I think. I've seen the same idea at work in a 5-minute set consisting of a joke-free lecture on how to back up your hard drive.
For me, that does mean having a tight script for a long bit. Even if I evolve the bit over time on stage, and never lay it back to paper again, I find it very helpful to have chosen a set of words as a starting point. I'm comfortable going off book when something happens in the room, but at the moment, I don't make a goal of doing any crowd work or of being the 15th open mic'er that night to talk about the stage.
I do find a couple short jokes or a shorter bit to get the crowd with you or clear the air from the previous act can help to get them on board for a long piece. I understand that's why some folks do the stuff about the room, but I prefer to stick with something more similar to the rest of the act.
I kind of think that most comics do, or start by doing, whatever seems easiest (laziest?) to them. Some of my pals that do 20 short jokes in a 3 minute set have said that my approach of writing a 3-minute bit, learning it, and then taking it around to mics and building on it from there seems like a huge amount of work. To me, burning up 20 premises and having to remember all 20 jokes seems like it would be harder. And harder to scale - when I started getting 10 minute spots, they were pretty much just three smaller sets chained together.