Tag names and spaces

[Dougie Nisbet <dougie highmoor co uk>]

What is the advice on spaces in tag names? Is it a good idea?

For example, if I create a tag name called Friends, and underneath add 
tags for each person, can I use Firstname Lastname, or should I use 
"Firstname Lastname" in quotes, or should a tag not have spaces at all?

Thanks,

Dougie