I only change it if the lever starts to have some sponginess to it. If you're comparing new to new with the only difference being the line-- I'd say that the limits of traction between the ground and the tire would be reached well before the limits of the plastic brake line could be a factor.
On a pavement oriented machine--- the steel line does offer a slight advantage, but moreso after the brakes get hot. I don't know that the brakes heat up enough due to the lower speeds of dirt riding. (less friction than the high speeds on pavement) I guess it depends on who you are as a rider (some riders are alot heavier than others too-- haha!), and what tracks you ride. I've seen "C" riders making brake line mods that they will get absolutely zero benefit from. I'd save the money to put towards another RM125 or RM250 if I were you.
Note-- this is coming from an individual with very limited riding skills, but who does have alot of experience with crashes most likely related to poor braking techniques over the years. --L*64