It's called "clean install" to uninstall the old version before installing the newest version.
As to the question, it's depending. If you trust the product and they make good updater/update patch, it's not necessary. Or if the uninstall and reinstall require hell lot of time and pre-patches (like most adobe, windows product) it's just too time consumption and not worth it.
some tips for you.
1.) test the uninstall programs - no matter how bad a software, they will try hard to make the installation clean and smooth, but not the uninstaller. If you find errors and problems on installation, you can know it's not as good a product as you thought.
2.) you don't need to delete the program directory or search window register yourself, there're many available freeware installer, like Advanced system care, CCleaner, they come with good uninstall tools to clean the junkie on your system "almost" completely.