My personal guidelines for letting Kodi off leash are that even though he's very reliable, he's never off leash near a road. Even a quiet country road isn't safe. In fact, they can be the LEAST safe, because drivers aren't looking for a small dog in the street, and conversely, you and the dog are both lured into a false sense of complacency. Every dog is still a dog, and I just won't take even the slightest chance that he would bolt after a squirrel and in front of an oncoming car. even in the woods, I always call him back to me and put him back on leash long before we get close to the road or parking lot.
In the woods, I really don't worry about squirrels. They go up the closest tree and the dog stops at the bottom.
I actually don't even try to stop Kodi from investigating squirrels and chipmunks. That's what he's off-lead for, is to enjoy being a dog!
When he was younger and I was first allowing him to be loose, I left a drag line on him. This was just a light weight leash or rope that I could step on and stop him, even if he wasn't listening. I don't think I ever had to use it, but it was a bit of an insurance policy. You never want a dog to learn that they can play "keep away".
But as Dave said, the biggest thing was just really, REALLY working on that reliable recall, so I could be sure he'd come back to me when I asked.