The fuel filter is not a simple screen. The pressure from the pump moves filtered out materials out of the main path of the fuel. So, as long as the filter is not old or heavily clogged, the price encured on fuel line pressure is only as material is pushed into the filter. So, the less percentage of stray matter that has to be filtered out, the less stresss on the pump. As the tank gets low, chemical differentiation does less to ensure that only gas is sucked in, therefore a higher amount of contaminates is likely to be present in the last few gallons of the tank.

Again, this is all speculation. I would love to see actual test results.