are negative x and y coordinates (i.e. -250, -250) the correct way to start actors off the screen?
Unless your actor has "Always Simulate" (Which makes the actor's coding work even when off screen), no. Either way negative is the incorrect way. At the far left of the screen, it's x 0, and top is y 0, so having -250 and -250 would mean it's way out of bounds. If you have an actor created at 0 and 0, he starts at the top left, basically. Then, the X number goes up as the actor goes more to the right, and the y number goes up the lower he goes down.