I never stop when I'm ahead and have NEVER regretted it. I always play level stakes and I know I will be having (on average over many years) 15-30 bets per Saturday (M,S,B,A,) . I only play Longshots ($20 or longer). This means a minimum,say, $300(15x$20) to a max $600(15x$20) risked per afternoon. If on the first bet you get a $21 winner and are immediately $400 up you could go home happy or you can STICK TO YOUR SELECTION METHODS. The next 14 to 29 horses dont know you've had a winner (and decided "enough is enough,we're not going to win any more for this guy"!!) so why would'nt you back them as planned?? At this point you might lose $280(the next 14 bets) of your $400 (still $120 ahead) or even lose $580 (29 more unsuccessful bets) and be $180 down on the day or ......... hit JETWAY in the 7th at Adelaide yesterday and catapult another $660 ahead (ie $20x $46-$20x13 more bets). Of course I still placed my last 2 planned bets (despite cracking tha "Grand") as the afternoon is NEVER over til it's over!!!
This reasoning goes even more so for shorties as stopping with a 2-1 or 4-1 winner when you are barely ahead means a great chance of giving that "puny edge" back next time. Plus: on any system or selection method or staking system or whatever that I've ever seen ,the profits come in clusters (ie several average or disappointing days followed by A BIG ONE ) This is the same with almost everything in life. That's the origin of the expression"Good things come in threes". Maybe they come in twos or fours but they DO come in irregular doses. What folly it is to be a the start of such a day and not to hang on to get them all. Cheers , and good luck.