It depends on may factors.
How clear are you from the field?
How many laps left in the race?
Is anyone chasing?
Is there wind, a hill, or other feature that would make one leg harder than another?
The last thing you want to do is blow up out there.
I try and do just a little bit less than the other guy. I don't attack, I always follow, because I'd rather take my chances in the sprint from behind. But that's just me. I bet RacerEx would do the opposite.