I use an EQ , really depends on what I am listening to, and what I feel may be 'missing' in a sense from my earphones/headphones.
I have the majority of my music on my Zune 120, which doesn't have an EQ, there's also that.
I listen to a relatively wide variety of music, some of it bass driven. Hip-Hop/Rap, funk, R&B, soul a little electronic, heavy metal/rock, jazz, samba, salsa, 'trailer music' (if that's the category Immediate Music falls under), some alternative, and I enjoy piano pieces, notably Fryderyk Chopin's works.
I do like my V6's sound, decent bass, the highs aren't too sharp (as it is for some) in my opinion, but I feel as if it is missing something, perhaps a little more presence in the low end, more impact, and it's highs and mids could use improvement, definitely.
To get a general idea of songs I listen to songs like Chopin's 'Ballade No. 1 - Op. 23 - in G Minor', Immediate Music's 'Lacrimosa', Rob Dougan's 'Clubbed to Death', Black Sabbath's 'War Pigs', The Killers' 'All The Things I've Done'.