See the discussion that starts with this post:
https://stat.ethz.ch/pipermail/r-help/2007-August/138145.html
Read through all the replies for the full discussion. Those posts refer to base graphics, but the ideas discussed apply to any tools for creating bar plots, including ggplot2.
The short message from the above discussions is that you should not feel the need to reduce useful and beautiful graphs to look like something from a spreadsheet, rather accept the wonderful results from ggplot2 without uglification. If you do feel the need to add additional information, then do it the right way that adds value to your graph rather than detracts from it.