Although catching this snitch is very important, (and in fact, necessary to end the game), as a seeker you need to know when to catch it.

Again, catching the snitch give your team 150 points. 

So, you need to make sure that if your are losing, your are losing by less than 150 points. If the score is Ravenclaw is beating Slytherin with the score 151 to 0, and the Slytherin Seeker catches the snitch, Slytherin still loses by one point.