IMO, backpressure IS always bad. Backpressure increases your pumping losses. The trick however, is minimize backpressure without sacrificing exhaust velocity. It become trickier yet when you realize that the amount of exhaust gas you're tuning for varies with engine speed.
I would have assumed that F1 cars were using short exhaust pipes due to the wide range of RPM's that they need to run at, while the Nascar teams can tune for specific RPM ranges that the cars typically run at. I'd also imagine that weight is a bigger factor in F1, and that NASCAR is supposed to be 'Stock' car racing, and rules would require a full exhaust system.
As far as using the exhaust as an aerodynamic aid, my guess would be placing the exhaust somewhere in the middle (taking a hint from Lamborghini here)
