Hey I use that for starters too... you can keep them for weeks. A little wort goes a long way. I boil and cool before using, then water down.
I use the 250ml@20, 500ml@20, 500ml@30-50 method. That is to say I water the first two starters down to gravity 1.020 and the last to about the same as the target gravity of the wort. Also you can add nutrients in starters that you might not want to add in the beer. Like zinc.
Edit: to be clear, you need the wort to be sterile and cool when you add the yeast to it. So even if things are growing in it while you store it in the fridge, boiling will take care of that.