I don't just delete them I use a program to do it for me.
The one I usually use it available here:
http://www.wisecleaner.com/download.html
It will tell you what is safe to delete.
If you have more than one partition and need to make one bigger you can use a partition manager like Gparted.
http://gparted.sourceforge.net/download.php
It is free open source software.
It has a GUI and is pretty easy to use but it is a really good idea to back up to some other media before you use it.
AG