It depends, what's important is that you don't want a bacterium or other organism taking over your substrate, which can happen easily especially with agar gel a

It depends, what's important is that you don't want a bacterium or other organism taking over your substrate, which can happen easily especially with agar gel and nutrient rich substrates. But if the mushrooms acquire a critical mass they can actually kill bacteria. There are many techniques, some emphasizing sterility. Others use a weak peroxide solutions which kills other organisms but not fungi. And others say sterility is overrated and just grow mushrooms in big plastic bags.