Once the spawn is fully colonized, it can out-compete other (mostly bacterial) organisms. But if you are not sterile before the spawn can gain a foothold, mold and bacteria can out-compete and ruin your spawn.
I take a full bag of spawn and dump it into a mixture of foods that I have pasteurized. So like HORSE shit and brown rice flour and other stuff depending on the mushies.
If the spawn is happy, and you put it into a nearly sterile media, you do not have to be overly concerned with contamination. You need good air exchange at this point anyhow because CO2 is the byproduct, and it will just hang out in your tub being heavier than air. That will cause new fruit not to pin. So unless you have a ton of equipment, the fruiting stage is not going to be sterile.
This is what I use for pasteurization of mass substrate
Based on a design by Myers Mushrooms, our substrate steamer is the easiest, most cost-effective solution on the market. Not intended for use with grain spawn. A typical cycle will take 14-24 hours. Time is dependent on ambient temperature, substrate temperature, and heater wattage. The 85...
www.bubbasbarrels.com
But for spawn, I use an autoclave under pressure to be 100% sure it is sterile. The pasteurizer is at atmospheric so I don't have an 85 gallon bomb in my garage.