portfolios and rebalancing, page-4

I have a written trading plan which states that when one stock becomes more than 66.666666....% I need to sell down that stock. I've had to do this with TIS a few times now. I would have probably been better off just holding the lot, however discipline I believe is important even for its own sake. My holding has moved down from 0.30368% of TIS down to 0.27657% of TIS as a result.

The stocks I've purchased with the proceeds of the TIS sell down have not done very well (yet; I'm still hoping in the long run the "discipline" will pay off!).

Does one really need a written trading plan to stick too?