I think this really depends on a lot of different factors. You said you live on the edge of town. Is your acre wooded at all, or is it a bare flat acre, so to speak.
My birds are not particularly adventurous, and i'm almost always here with them. They explore around the yard (1 1/2 acres, largely wooded) a couple of times a day, but most of the time they like to stay near the woods and in the shade. So i don't really worry about Hawks since by birds spend a lot of time in the brush.
I also only have three. I think that makes a difference too.
There are just so many factors that i really think you have to make a judgment call based on your particular situation.
If i were not at home for 8-10 hours a day, i would definitely not feel comfortable free ranging my birds without having at least a fenced in portion for them to be limited to.
I hope that made sense.