Always. That is the best time to do gravity adjustments. You can add water or lengthen the boil without screwing up your hop additions. You can also add sugar or malt extract to make up gravity. Once the boil is over is too late to do much without risking messing things up.
It's good to know where you're at in relation to the predicted numbers. I'll also add that a refractometer makes this so easy there's no reason not to do it.
"The largest room in the world is the room for more beer." -Nelson Mandela
