The difference between labeling yourself as an atheist or an agnostic is primarily political. If you want to take a position of advocacy against religion, call yourself an atheist. If you don't want to take a position — and I don't feel anyone is obligated to do so — call yourself an agnostic.
Leave the epistemological hair-splitting to the philosophers and theologians. If you're interested enough in philosophy to contribute to the hair-splitting, then your choice of label is irrelevant.
There are some bad reasons, though, to call yourself an agnostic, reasons that fundamentally insult those of us who call ourselves atheists. If you say, "I won't call myself an atheist because I don't believe X, then the clear implication is that those who do call themselves atheists do believe X. If atheists don't in fact typically believe X, then you're insulting atheists by attributing to them a belief they don't typically have.
I'm an agnostic because I'm not certain that God doesn't exist.
This is stupid. No skeptic is certain about anything. Everything is uncertain. Everything is subject to revision... even the notion that everything is subject to revision: for all I know, some genius philosopher (or theologian!) will prove tomorrow that we can indeed be absolutely certain, beyond even the possibility of error or omission, about something. Until then I'm not going to make any distinctions on the basis of certainty.
I don't believe that God exists, but I don't believe that God doesn't exist.
This is bullshit philosophical hair-splitting. The only reason to draw this distinction is to say that "God" is always completely meaningless. Now it's true that a lot of people use "God" in completely meaningless, incoherent and often ridiculous ways (e.g. "God is not a member of any set") but millions of people mean something very specific by "God", and they're wrong: There is no such God as, say, Yahweh or Allah, the characters depicted in the Judeo-Christian bible and the Koran.
Honestly, if you take a completely meaningless term, such as "gnort", does it really make all that much difference if you say, "I don't believe that any gnort exists," or if you say, "I believe that no gnort exists?" If you think it does make a difference, you're studying for a theology degree.
Atheists are dogmatic.
If you think having any definite belief whatsoever is dogmatic, then this line is for you. If you think it's OK to have a definite belief about what's actually true, then please judge atheists on whether their beliefs are actually true, not that they have definite beliefs.
Dogmatism is believing some statement with certainty (see above) or on the basis of pure authority. If you believe that unbaptized infants go* to limbo because the Pope says so — and on what other basis could you possibly believe such a ridiculous statement? — that's dogmatism. To believe that things fall when you drop them, that the earth is round, that people and apes (and roses) descended from a common ancestor is to have definite beliefs that are actually true.
*or don't go; I don't keep up with ever-changing eternal Catholic dogma.
It's very simple: If you think 100% of human God talk is lies and bullshit, and you object to lies and bullshit, then call yourself an atheist. If you're not sure, shut the fuck up and listen until you can bring yourself to make an actual decision.