How can I add a scale reference to the Z axis Blender has a convenient reference grid with 1-unit squares, centered on (0,0,0) in the X-Y plane; I want something similar on the Z axis.

I know I can add reference images to the background, and I could create one manually for this purpose ... but having to go to a separate program and import a specially created image seems excessive when I just want a simple ruler to eyeball things against. My current workaround is to add several plane meshes to make a vertical line of squares that I can measure against - but again, it feels like there should be an easier way to do it.

Is there a quick and easy way to do this? Ideally, it would let me set a custom interval for the gridlines/ruler ticks, but I'd accept the standard 1-unit ticks if that's the best I can get.
Easiest option

Orthographic views tend to be useful for this. Even if you're eyeballing there will be less distortion in the perception of location. Orthographic views will show a grid in all three orientations. -- font, left, top ... and their reverse: back, right, bottom. See the Blender manual for all options, and menus relating to this



Here Front, Top and Left are visible. 

possibly interesting option

An addon called 'Chromoly Ruler' or just 'Ruler', is included in the addons_contrib. If you get blender from somewhere like builder.blender.org. It looks like this