The reason they tell you not to drive at constant speed is that accelerating and decelerating while in gear puts the maximum range of pressure/vacuum loads on the piston rings/cylinder walls. Going smoothly from WOT to zero throttle in your cruising gear is the easiest way to accomplish this while keeping the engine in a mild rpm range. Again, I've heard so many different explanations of why this is desirable (or undesirable) that I can't really comment on that part--just that that's the explanation for not driving at a constant speed.
Honestly, unless you have a firm conviction otherwise (which it sounds like you don't... lol), I'd just follow the manufacturer's instructions and not worry about it. If there was a definitive "best way" to break in an engine (or if it really mattered that much), there wouldn't be so many of these conflicting theories floating around.
On the way to the opener I floored it and my gas pedal got stuck under my mat!! Whoever put my floor mats in didnt use that little hook... That thing is useful! Bounced off the rev limiter when I restarted it(before I noticed what was wrong)