My preferred approach is to provide a means to specify the CSS in the plugin settings along with a "use these styles" option. I also like to provide full documentation on what CSS classes and ID's the plugin utilises. This gives the end user full control of how the plugin output looks.

No matter how configurable you make the CSS for your plugin, you'll never be able to take into account all the layout's, colour schemes and user preferences out there.

Default CSS should be basic and should take into account different browser implementations - don't include browser specific CSS styles.