Here's what I think I know, but I'm a geography newbie, so please correct any misconceptions I have, especially with regard to the term graticule.
A graticule is a specialized grid where the lines represent parallels and meridians. If those lines are latitude and longitude, then this kind of graticule is called a "conjugate graticule".
This is what I think is the most common usage of the term "graticule". However, I think it's just a common usage (and not a necessity) for the graticule's lines to be latitudes and longitudes.
My question is, if you have a UTM based map, isn't it possible (and more useful) to have a graticule in UTM coordinates (meters)?