Attached is a screen capture that shows the same dashed line symbol being used to show county boundaries. The top image shows a relatively clean boundary drawn from a line feature class even though the lines aren't straight. The image on the bottom shows the same bondary using the same line symbol but this time drawn for a polygon feature class. In this one it doesn't even look like a dashed line. It looks like ArcMap is drawing over the line twice, once for each polygon.
Is that some setting under symbology in ArcMap that I'm missing? I'd rather not convert all polygon feature classes into line feature classes just to get a "clean" dashed line symbol between polygons.
If: 1) you don't want to or can't go the representation route, 2) you don't mind a performance penalty which could be almost unnoticeable, and 3) you don't mind that your line will block out any line under it, you might make your outline a combination of two lines, one a solid line with the color you want to see and the top one a cartographic line of white color with a template pattern that when combined with the solid produces the pattern you want.