In looking at the above responses, everything from 'no head' to 'single axis' to 'ball head' is used. It all comes down to 'whatever works best for you'. Having the ability to choose which, if any, head to use is perhaps the best option, as shooting situations vary from location to location.
For what it's worth, when I use my monopod, it's usually in low light situations in church (no weddings) that require the added stability needed for slower exposure times. So, I find myself often leaning against a wall or pillar for stability, in addition to the mono pod which ends up leaning inward towards me slightly. As a result, I use a fairly heavy ball head that gives me complete control of compensatory adjustment in any fashion to keep the monopod on secure footing and the camera aimed where I want it. A single-axis head just wouldn't work for me.