I learned only after losses of course, if the stop is within 20% of the stock, never ever have it in before the open, how many times have you been stopped out at lows for the day, week or month cause you became your own worst enemy? Waiting five minutes to let market settle down if it gapped down and then stop is placed below the five minute low, this little more risk than being out and cursing watching the market reverse.
I have a procedure that I use and teach for the determination of both stop-loss and profit target levels. The procedure is based upon that particular spread combination's historical trading range. I demand that my clients set and enter GTC limit orders for both the stop-loss and the profit target at the time of trade entry. It may seem a bit trite and old-school, but it has been working quite well. I have a responsibility to my clients to do my best by them, and to keep them alive. The holy grail is that there is no holy grail. But the closest universal skill for a successful trader is sound position management. And I find that traders using trade-by-trade discretion can have problems. We are in the business of calculated risk. Just my 2 cents, YMMV.
Market conditions may hinder a stop order from being executed exactly where you place it. That is why you should consult with your broker on their written policies and details of how they execute stop orders.
The greater size of the Forex market gives it greater liquidity due to the numerous traders that are trading at any given time.
If you are participating in a really thin, volatile market like Cotton or Gasoline or certain metals futures, you are going to have to specify an aggressive stop limit range - that's the nature of those markets. If a wide stop limit range is unappealing to you, then consider liquid product names or exchange supported spreads.
I agree, that is why,unlike a market order, traders are advised to use limit orders as well. A limit order only fills at the price you want, or better.
Point number three has cost people a lot of money. Google "flash crash" of 2010 I think. A stop loss does not guarantee you will get out at the stop loss price, it does guarantee you will loose money in a crash. Better to use good portfolio allocation and keep your eye on the ball. SMS alerts can help. Does anyone know, if you put in a limit order to sell a stock, does the limit just get converted to a market order when the price is right, or are you guaranteed to get the limit price or nothing?