Now seriously, I only find useful to prepend www.* to my domain when I have plenty other subdomains, as for instance a shop, a landing page (it'd be the www), a blog and so on. If I only buy a domain to put a web (portfolio, blog or whatever), I just leave it as it is. Maybe it's "wrong" regarding some defacto rule, but I usually find it redundant when in the second case.
when on the phone, I like to to tell people to go to 'whoo whoo whoo' then my website domain, because that's a how a teacher in school used to talk about web sites in class in the 90s and find it funny.