I risk only the amount that I can easily afford to lose. Every trade is random, no one knows what can be the outcome of it so trade safe and risk less.
If one is following a set strategy their trades shouldn't be exactly random. However, I do agree that one should always keep in mind that the market might surprise you, and sometimes those surprises are can be unpleasant, so it's best to be prudent about much of your account you risk when you're trading.