In my view, it's better to sell when there's a reason to sell, rather than to cap your gains at 8%. I'm assuming you have no such criteria on the other side - i.e. hold your losses down to 8%. That's because what matters is how much you make overall in your portfolio, not how much you make per trade.
Example: if you own three stocks, equal amounts - and two go up 20% but one falls 20%. If you sell your gains at 8%, and hold the loser, you have net LOST money.
So when do you actually sell? You might say a "fall of 10%" from the last high is good enough to sell. This is called a "trailing" stop, which means if a stock goes from 100 to 120, I'll still hold and sell if it retraces to 108. Needless to say if it had gone from 100 to 90, I would still be out. The idea is to ride the trend for as long as you can, because trends are strong. And keep your trailing stops wide enough for it to absorb natural jiggles, because you may get stopped out of a stock that falls 4% but eventually goes up 200%.
Or sell under other conditions: if the earnings show a distinct drop, or the sector falls out of favor.
Whenever you decide to sell, also consider what it would take for you to buy the stock back - increased earnings, strong prices, a product release, whatever. Because getting out might seem like a good thing, but it's just as important to not think of it as saying a stock is crappy - it might just be that you had enough of one ride. That doesn't mean you can't come back for another one.