It doesn't really matter as long as you aren't slipping. They always try to put you at the very top of the roller. My background in physics tells me that the work associated with turning the roller will not change if the tire is not at the peak of the roller. The torque arm is the same.
I could be wrong. It's happened many times in the past.
Well considering its a round drum your turning i dont think it really matters where you place the car. But you do want to have it so the most force can be applied to the drum from the wheels and holding the car to the dyno imo is more important as to prevent any wheel spin to get correct data. you want the wheels to maintain contact to get the correct reading and placing the car too far from TDC could make it easier for slippage.
Like others have said not sure that it matters. Having said that, the guys who dyno cars at 21CMC back the car onto the rollers and go just past the top of the drum and strap it down. Under load, no matter how tight you strap it down the car will move forward slightly so that tire climbs the slope of drum. Probably provides more traction.
Your putting a round object on top of a round objuct. Even if you put it at TDC its going to move some either forward or backward since the weight of the car will shift and the roller is designed to move (obvoiusly since they have to spin to dyno).
It only matters w.r.t. traction. Provided you aren't slipping, location is inconsequential. I could imagine that being slightly in front of TDC would potentially improve traction by allowing the wheels to "squeeze" down on the roller under load, but as far as the physics goes, torque on a Dyno has no concept of "angle".