What is the proper timing on a subbed video?

-I am working on Subtitling a video and I was wondering if there was a certain timing that was best for subtitles. I have it set to where it starts just a few seconds or less before you hear the words. But on a lot of other subtitles I have seen most of the subtitles start a good deal amount of time before you hear any words. I was wondering if that was the best way, or if it doesn't really matter?