As ColinB already said, the fuel pickup point in the tank is fixed, it does not float around - so however much fuel in the tank the petrol is being drawn from same spot, so if there is dirt there with an almost empty tank, there would have been dirt there with a full tank.
But this ignores floating debris.
Read the previous comments. SteveiD said:
Best not to run to the bottom of the tank as there will inevitably be a bit of dirt or even water down there!
That refers specifically to water or dirt collecting at the bottom of a nearly-empty tank, and perpetuates the myth that those contaminants will somehow evade the pump until the tank's nearly empty. In reality anything in that position would already have clogged the filter or been ejected via the engine because it would have been first to get sucked into the pump, not the last, because that's where the pump inlet is. The point is that it's not somehow worse if the tank's empty. We both pointed out that the only thing that would concentrate at the bottom of a nearly empty tank would be floating debris as the free surface gets closer to the pump suction.
And, personally, I wouldn't rely on the range calculation to judge how much fuel is left, you only have to look at how that number jumps around as you drive.