We had our first dogfight the second night our second foster was here- woke up to bloody murder at 6 AM, accompanied by a quarter-sized chunk out of the foster's throat that eventually became infected. (Memo to the newbies- don't make my mistake. Don't just bandage- clean with Betadine, and use the best non-stick you can find for on top of the wound. Clean every 8 hours at a minimum.) She's fine now- you can't even see the scar, but there's tail-flagging all the time, and intermittent squabbles between the two. (Second memo: It's like roller derby with greyhounds. It's the females that start the fights, in my limited experience.)
So, now when we leave, everyone gets muzzled. Two of our pack can't slip the muzzles the way they are, but there are two others- the fosters- that I'm not convinced about, so the muzzles get tied into the martingale collars once I slide the collar up as high as it can go. Before that, we crated; we abandoned that once we figured out they were crate-chewers, even though we never saw it. One ended up chipping a lower canine- an expensive proposition that required extraction.
I still don't muzzle in the backyard; to date, there have been no squabbles there- it's always been over sleeping spaces, even though there are more than enough blankets, pillows, and floor space to go 'round. We may revisit muzzling in the yard, although it's a rather small yard and normally they just stand on our porch to get let back in when they're done.
When we go to the vet, every grey is muzzled. Three of the four have very high prey drives, and when weiner dogs and cats look like snacks, we position the greys as best we can in the waiting room. For our annual checkup, I got them the first possible time in the morning to prevent any run-ins.