We use the Halo swaddle/sleepsack on DS. the Halo allows us to swaddle arms in or arms out. DS is a houdini so he gets his arms out pretty often anyway. However, for the last few nights, we purposefully swaddled him with one arm out and one arm in. FWIW, he slept even longer the past few nights w/one arm out!
We decided to leave his arm out (R arm) because he fights the swaddle so much and he's gotten so strong that his R arm is lifting the swaddle part up - essentially pushing it over his face. I don't like that and that's why the de-swaddling.
Oh, and after MOTN feed, we leave BOTH arms out. DS goes into light sleep mode towards the morning, which means he struggles more against the swaddle. When we leave both arms out, he doesn't have to struggle and is less restless.