It's said that Timo Tolkki, guitar virtuoso for the power/speed metal band Stratovarius, gets the title for his tunes before he even writes any music. When I learned this, I felt like it explained the majesty of songs like "Eagle Fly Free," and the melancholy of anthems like "Destiny."
In a lot of ways, I'm the same way with writing. I think of one concept, item, or phrase, and it attaches to the idea I'm developing in its entirety. In this way, early ideas greatly impact the early instantiations of my work.
But later on, I usually end up changing the title, as a theme develops and I see other elements of the story emerge.
For example, I just tossed the old title of my novel-in-progress, "Panopticron," in favour of a new one: "Remanence." Both of those terms are actually pretty clutch in relation to the development of the story: it's a steampunk-fantasy, and the vessel called Panopticron, and the magical power called Remanence, are both key to the way the tale plays out.
Difference is, Panopticron - which, lets face it, sounds totally cool - is in no way metaphorical, no way magical, no way greater than itself. Panopticron is pretty much the sum of its parts, like the machine it is in the story.
Remanence, on the other hand, is a cool little electrical property, and provides a great metaphor not only for the function of magic in the realm I'm detailing, but also the way in which people think about stuff in this world - what passes for a post-apocalyptic one, in a fantasy realm. "Remanence" isn't just about magic; it's about the vestiges of divine grace that a motley assortment of people are groping at in a dying world, and that relates back to a world - our world - that, to me (and this is only a personal opinion, but personal opinions are important for enjoying your writing, aren't they?) feels somewhat hollow as the "true miracles" of science replace the spiritual ups-and-downs of yester-millennium.
Titles are really important for writing, whether you come up with them before or after. At the end, they cast your story in stone, in effect sticking a flag on top of the castle that - until someone starts digging - is buried in sand. But for me, as a writer, it's incredibly important to have a working title all along. I want to know what I'm digging for, because if I don't, I might dig in a direction that uncovers something I'm not particularly keen on.
Say, a cavern full of brain-eating trolls with six legs. Great story fodder, sure, but not when my vital organs are a staple of their diet...