Depends, I rewrite alot of my old scripts that I'm not real proud of how they were written. This is known as code refactoring.
Code refactoring is a "disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior", undertaken in order to improve some of the nonfunctional attributes of the software. Advantages include improved code readability and reduced complexity to improve the maintainability of the source code, as well as a more expressive internal architecture or object model to improve extensibility.
By continuously improving the design of code, we make it easier and easier to work with. This is in sharp contrast to what typically happens: little refactoring and a great deal of attention paid to expediently adding new features. If you get into the hygienic habit of refactoring continuously, you'll find that it is easier to extend and maintain code.
—Joshua Kerievsky, Refactoring to Patterns
So, personally, I think if you have time, it's a good thing and a worthwhile thing to do.