Most all the standard footprints for a QFN already extend enough past the part to get a bead on the side pad. If you extend it farther, all you do is open up the possibility that you can solder the part on so far out of whack that the center pad can bridge the other pads.
If you do not have to solder the center pad, it is eaiser to leave it bare and to drag solder. Flux the pads, drop the part down, tack one pin, then drag solder while pinning the chip to the board with tweezers. If the chip looks crooked and/or if you can't get one side soldered because there's not enough pad sticking out (i.e., you put the chip on crooked), just solder the 3 sides that work and then hit with hot air; at the exact moment the solder reflows, the chip will spring into place.
If the part does not self-align, it's most likely because you got a wee bit of solder on the center pad.* When this happens, the part sticks to the board and isn't going to move. So after X amount of time, if the part isn't springing into place, it's probably not your imagination. It's probably not your hot air station. Try pulling the chip off, tinning and fluxing the pads, and putting it on with hot air.
*For this reason, if I'm soldering the center pad (using hot air), I prefer to err on the side of using too much solder than too little. Any excess, you can just squish out by pressing down on the chip after the solder reflows. But too little solder can cause a larger problem.
Personally, I prefer a tip with a sharper corner/edge than a chisel tip. Knife or bevel tip have a sharper corner and will get deeper into the 90 degree corner between pad and the very shallow sidepins. The bevel is actually better because it fills the 90 degree angle pretty much perfectly; the angle on the knife is too acute, so you have to hold it at just the right angle to get the joints to form. That said, I prefer the knife when doing batch, because it will hold enough solder to do a lot of chips in a go. And in batch soldering, once you find that specific angle, it is easy to hold it and do multiple chips in a row. For batch soldering, this is another reason you do not necessarily want to extend the pads out. You will have to load the tip more frequently and/or feed solderwire with your off hand.
So you will want flux and hot air if you are dealing with QFN much. Even when hand soldering them with an iron, you will want extra flux. And you will want hot air for when things go wrong.