On your entire portfolio? I personally never use stop losses because I am in front of the screen the entire day. I have found that sometimes mm's see stop losses and bring the price down just to get those shares and then they go back up again. But, if you are away during the day and can't watch, it is a useful tool.
You have to decide personally what type of risk you can take and for how long. Are you long term? Are you talking investment or trading here? I take losses early and at small percentages, but I trade very often. If you are long term it is totally different. It all depends on how much of it you need near term, your age, etc.
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