Most limb spreaders wouldn't harm a tree if you left them in there til doomsday. However, any tying with wire or twine means girdling (this coming from a seasoned girdling pro).
Furthering the discussion, I tie them down all year long, as it is often not a priority most days. When the limb is in place, cell development will help lock it in place, so earlier in the season of course makes sense. A bent piece of wood will always maintain the shape to an extent. Since it is positioned, remember next spring/summer growth with help lock it in. It goes without saying that after leaf drop it is a whole lot easier to see what you are doing, and post pruning even easier.
Two things: Cold usually means brittle, so be careful with pushing things too far (pears are the worst). Twisting branches as you bend them will help prevent snapping them. This latter phenomenon is how basketmakers and medieval fencemakers made the turns.
Walden Heights Nursery & OrchardZone 3 in Vermont