If you read some posters on the open forum, they claim everything from sewer flies to alligators will climb up your drain hose and into your tank/coach. I've been RVing for 43 years, the first 30 years, I used a 3" drain hose and always left the grey valve open all the time and never had an issue. The last 13-14 years, I've been using a SaniCon macerator. I never worry about flushing the hose with grey water or even flushing it with fresh water, unless I'm parking the coach for a couple of months. That's the beauty of the SaniCon system. Dump and cap the end.
If you have the SaniCon installed, it has a grey water bypass. With the original unit, it was a small 3/8" hose that let water trickle through. It was pretty much full of water all the time, not letting anything pass up stream. I haven't torn a new SaniCon unit apart, but I'm guessing it's still tough for something to crawl or fly up it.
As redbraron73 stated, it's an inconvenience, at least for me, to not be connected with the gray valve open. I never worry about flooding or full tanks at an inopportune time.
Don & Mary
2019 Newmar Dutch Star 4018
2019 Ford Raptor