Adding to winners works way better in discretionary trading unless you're a _very_ disciplined and meticulous person who also have properly researched what you're doing. Would only add to losers in thoroughly backtested automated trading (and with martingale aspects properly accounted for).
All the guys I’ve seen who got fired by their electronic prop firms or who had their Accounts closed and floor trading clearance revoked - just about all of them added to losers.