If you hold
Ctrl you can restrict the angles of the lines (I think you can change it in Preferences but the default is 15 degrees), this way it's very easy to draw vertical and horizontal lines.
In development builds Pen tool has an additional mode that only lets you draw horizontal and vertical segments (you can have both official 0.46 and development build installed at the same time if you want to try it).
Grid with snapping on helps a lot, especially with
Ctrl pressed
To get angled lines at exact angles you can change the preference to the angle of your choice, or use angled guides or Axonometric grid (you can change the angles in Document properties > Grid tab).
There is an effect that matches the arrows color to the line color, look under Effects menu.