I've experimented with a beverage thermometer, and even preheating a pot or mug, it's impossible to get the temperature to actually stay at the boiling point. The only way to do this is to actually throw leaves into a pot on a heat source like a stove, while the water is at a rolling boil. Some cultures do this, but it's usually not the best way to brew tea, because a lot of the aromas are lost to the air.
But this doesn't really matter. When people say to use boiling water, they don't mean that it has to be exactly at boiling during brewing, just to heat it actually to the boiling point. In some cases, with more sensitive teas, and especially if you're in a cold room and using a brewing vessel that holds (and thus absorbs) a lot of heat, pre-heating can be important, and can result in better flavor.
I've even noticed dramatic differences with Lipton's plain black tea in tea bags...I think it tastes a lot better when you pre-heat the brewing vessel and use water taken. That said, it's still not my favorite tea. =) But I do notice it comes out better when you pre-heat and use very hot water to brew.
Experiment though. You'll find that pre-heating matters for some teas and not for others, for some brewing vessels more than others, and probably find it matters most when the room is cold. And you may find that you prefer to use slightly cooler temperature for some black teas (I often hear this about Darjeeling first flush, although I usually don't agree personally). And you may find that your results or preferences aren't the same as other people's, because brewing is a matter of personal taste.