Late goals and dramatic declines

How many goals do Manchester United, Arsenal, Chelsea, Manchester City, Tottenham and Liverpool score in different segments of games? I would have thought Man Utd would score most of their goals in the first 10 minutes and the last 10 minutes of a game, as they always seem to come out and try to get an early goal, and they have that annoying habit of getting late goals. I don't understand why teams don't ensure they are switched on from the first whistle when they play Man Utd - should they be more wary?