I understand that you'll tie/fence your dogs, so I'm not so much directing the following at you, but for the purpose of conversing on the general subject of fence or no fence.
I'm never sure any distance is enough with dogs. When they catch something in the wind, or get the bug to wander, they can go places.
My folks have 30 hilly acres, with plenty of space between them and their neighbors. Still, my dad's dog managed to travel about six miles and wound up at a neighbor's place. Of course, this story has a sad ending because, even though these people knew the dog {and in fact, my dad let their cattle graze on his land for free}, they shot him anyway/
I'm NOT saying your neighbors would do that; it was just how this particular travelin' dog story ended. The whole point was simply that there just ain't enough acreage to guarantee a curious dog will stay at home and/or not be injured somewhere.