Quote from Cutten:
Therefore anyone who pursues a policy of adding to a winner, or cutting a loser, solely because it is a winner or loser, is being totally irrational. A position should only be added to if the current price & odds favour a bigger position. A position should only be cut if the current price & odds indicate that the trade sucks.
Adding to a winner based on the fact that it has a trending tendency is favoring a bigger position when you are on the right side.