Each combo is different, but I am a fan of loose. I picked up about .05 going to a much looser converter, 6400+rpm compared to 5500rpm before in front of a 528 BB with peak torque at 5100 rpm. Each combo will differ, and those chasing the best ET will have more than one converter to test.
For what it is worth, a loose converter will have less hit in the first few feet, but average better to the 60 ft clocks. If you have a situation like mine, too much launch torque, then loose makes more sense. Especially if there is spin off the line, which starts the lockup process early and decreases torque output to the input shaft. If you are allready way above max torque, it only gets better with a little slippage. Check out how the ET falls off on a car with a tight converter (roughly at or below peak torque for flash) when they experience slippage at the line.