I don't think any good BJ mentor would suggest that you quit while winning. In my way of thinking, win goals or just stopping in the middle of a good run simply adds to the house advantage. Most players that play a reasonable amount of blackjack (4-6 times a month or more) average out at some point inorder to take stock of how they are doing. So what does that have to do with the HA? If you expect to win, your winning sessions must exceed your losing sessions. One good winning session can offset 3-4 losing sessions and knowing that one simple fact should make it very clear as to why you should not quit while winning. Artifical goals is one way of saying that I can time my results, when what I'm attempting to time is the great unknown.
To my way of thinking the key is to not set a cap on how much you win so when that can't-lose night comes you're ready for it. At the same time nothing is more disheartening than getting (in my low-roller, $10-table case) several hundred up and then giving it all back (and sometimes more). I try to use a sliding scale. Normally my buy-in is $200 and, for the most part, when that's gone, I'm gone. If I'm winning, I always try to keep that $200 buffer. In other words, if I get up $100 at any point, then the most I'm going to lose for the session is $100. If I get up $200, the worst I'm going to do is break even, if I get up $300, the worst I'm going to do is walk out $100 ahead. You get the picture. At no time would you have to quit during a hot streak. I wish I could say I have ALWAYS stuck to that plan religiously, but, you know ....