my pup RIot is 4months old now and although i'm sure he'd LOVE being outside all day (minus the rain today lol even though he thinks rain is fun..) I could never leave him all day out there. I Do know lots of dogs do it, but you do have to be careful with a boxer because they do have a very short coat and get cold easier then other dogs.
A friend of mine has a pure boxer and HAS on occation left him outside in his fenced in "environment' which consisted of a huge water bucket for him, a dog house, huge dog bed and blanket plus toys. He stayed in there until he figured out how to escape lol but in the winter and cold weather he wasn't left out there.
I'd be more scared of someone stealing my dog, or it getting out of the yard.