dot products

When coding for a dot product, I can get away with just doing vectorA.x * vectorB.x + vectorA.y * vectorB.y, without bringing a cosine into the mix, can't I? I'm trying to write up the collision-method outlined in "Getting to the Point," here:

http://www.gamasutra.com/features/20000210/lander_03.htm

I like this method, because it could potentially mean that I don't have to deal with rotation at all, anymore. Any help with this little hurdle is very much appreciated.