I have seen my share of collisions and excessive contact in ultimate over the years.
Most of my time out of action has been from injuries sustained in collisions.
I believe you should avoid unnecessary contact in ultimate. In many cases it is a foul. In the other cases, it still probably puts people at risk of injury.
One example is a dump cutting up the line for the disc, while a poaching defender from upfield comes in for an intercept. The dump usually cannot see the defender, while the defender usually can see the dump. I feel the defender has more responsibility to avoid contact here. Personal experience: as a dump, I have been slammed in the back by a defender who I never saw, as I was looking at the disc. They claimed they got the disc first, so all was ok. Also, I have been the poaching defender and bailed on bids because I couldn't get past the dump without a collision.
Another example is a player running past the thrower from behind. If the thrower suddenly pivots sideways, there can be a collision. All players should keep a safe margin between themselves and the thrower as they run past from behind.
Parinella, veteran of the US scene, multiple title winner, etc, reported this in a recent post:
"Oh, I also took out a pivoter. The lanky opponent had caught a disc near the line and I thought he may have been out, so was thinking about that a little as I jogged downfield with my guy. I think I followed in my guy’s steps, a foot or two outside the pivot foot (but six feet from the thrower’s body), only to be surprised by a rapid pivot back to the forehand side, and I bowled him over. Whoops. I should have been more alert and known that was a possibility. I apologized to the guy on the other team who yelled at me about it, to the thrower, and again later to the thrower, and now to the unwashed masses."
So a highly esteemed player publicly apologises for creating a collision. He felt responsible because he was sighted, and the other guys wasn't. Kudos to Parinella here.
My general principle is that if you can see the possibility of a collision, you have a responsibility to avoid it.