Viewports have a property that allows them to be "locked", so they can't be re-scaled or panned. While this can be a little obnoxious on detail sheets (where things tend to change size a bit more), it's invaluable on plan sheets. Especially if you tend to double-click in viewport to make a small change, then zoom before you remember which space you're in. (It's happened to all of us.) Make your viewport, lock it, and you never have to worry about the scale again.