same thing happened to me and for me its not really a matter of having the crack hurt the screen its the fact the crack is there. it is very annoying to see the crack but I don't feel it's worth repairing.
If you really are worried, place some clear tape over the Apple to ensure that the plastic doesn't move further towards the LCD and damage it. Also, a protective case over the back of the Aluminum might help deflect any further pressure.
Since the cost to repair the plastic is basically as much as repairing the entire LCD - you might save your $$ until further damage happens.