First, there is an overarching principle that needs to be understood: avoid any type of redundancy on a slide. It's easy to determine where the units should appear if you adhere to this principle.
For example, if you have several data-driven charts on your slide, but they're all in the same unit of measurement, the unit would be placed under the slide title and not under the headings for each data-driven chart. No redundancies.
If you have several charts on the slide, each with a different unit of measure, then the unit of measure would go under the heading for each chart and there would be no unit field under the slide title. No redundancies.
If you use symbols for the units of measurement next to the value amount, no unit of measure text is needed - the symbols do the work for you, i.e., "%" or "$." No redundancies.
If you have axes and axes labels on your chart, the unit of measure goes under the axes headings, unless the values have unit symbols. No redundancies.