That depends entirely on who you talk to. Generally there is no need to unset variables unless keeping them around is particularly expensive. For example, if you loaded a huge XML document (talking tens/hundreds of megabytes), images (GD), etc. into your script.

Just like the common advice is to close any open database connections (and free results) as soon as you're finished with them, it's a good idea to tidy up a bit but don't go over the top by unsetting everything.