Tableau employs good practices for formatting and visualization from the time you start dropping fields on shelves. Tableau 10 additionally introduces a variety of new fonts, colors, and defaults that emphasize aesthetic appeal. You'll find that the discrete palettes use colors that are easy to distinguish, fonts are pleasant, grid lines are faint where appropriate, and numbers and dates follow the default format settings defined in the metadata.
The default formatting is more than adequate for discovery and analysis. If your focus is analysis, you may not want to spend too much time fine-tuning the formatting until you have moved on in the cycle. However, when you start to consider how you will communicate the data to others, you will need to contemplate how adjustments to formatting can make a difference in how well the data story is told.