You have a line whose length you want displayed at all times, right? Your solution is to put a line label on it that displays the length. Make your line. Option-double-click (*) on it to create a line label. Use the text <%length%> as your label (or use Edit->Insert Variable->Line Length). Now you'll always have the length of that line displayed, even if it is at an arbitrary angle to the grid. You can control the orientation and behavior of the label in the Properties: Geometry inspector.
(*) Default setting, see Line Editing section of Tool Preferences in OmniGraffle preferences.
