Danger, the neighbors' yards are always o.b. But, its tough sometimes to tell where the neighbors' yards stop and the fairway begins. In a tournament, the OB will be run from the last fence post along the inner (south) edge of the trees closest to the fairway. That way there is no question that we are not playing in the neighbors' yards. The string or paint runs from the last fence along the inner edge of the trees until you get to the yard beyond the pin.
In casual play and league, we use a line from the last fence post along the outer (north) edge of the trees (closest to the houses), which is more in line with the property boundary; but, also gets closer to the neighbors yards. We treat it as if there is an imaginary line running from tree to tree. As long as you are inside the tree "lines" on each side of your disc, you are in bounds.
If you throw a roller, you don't need to worry about ob anyway. Although, I doubt you have the guts to even try a roller on that shot!