we use words to communicate thoughts, feelings, actions....so when i think about using a label to describe someone or something, i consider whether i think it will tend to obscure or clarify my thoughts/feelings/actions. and it almost always depends on the particular circumstances, people, and ideas involved in the conversation.
in my experience, using labels to describe people usually tends to limit understanding and restrict the possibilities for conversation. but, if i intend to incite reaction in another person, rather than have them understand me, a label might work well.
over the years, i've had people refer to me as yuppie, hippy, liberal, libertarian, radical, moocher, oblivious, intellectual, anarchist, freeloader, visionary, utopian, and more, including the titles of jobs i've worked at. i reject them all and can't remember a time when i referred to myself as any of those things, other than anarchist (which i've used only recently, and even that label i don't find useful much of the time), and my job titles (which i've now let go of almost completely).
i figure, if someone really wants to get to know me and my thoughts and desires (and if i want to know the same about them), we'll spend a lot more time and energy talking and doing stuff together.... and that allows us to communicate and relate much more than a label does.
edited to ask m24wolf....
when you ask "should" (someone refer to themselves as radical), what do you mean? to accomplish a particular objective...or as a way of relating to someone...or for some other reason?