A magazine signed me up to do an article recently and offered me $375. That's not much, but it's a straightforward article that won't take me much time, so it's worth it. I asked if they could pay me $425, and they came back with a counteroffer of $400, which is what I expected them to do.
It never hurts for a writer to ask for more. I always ask for more from magazines and book publishers, and the worst that's ever happened is they've insisted on their original offer. Some beginning writers are afraid to do this, but any editor worthy of the name isn't going to dump you just because you asked for a bit more money.
There's a technique to doing this right, though. I never ask for a huge amount more than their first offer, and I don't pester regular markets unless I haven't had a raise in a while. I also ask politely, something important in any type of business (and writing is a business, much as I hate that fact). If one of my book publishers won't budge on the amount of the advance, I'll try to renegotiate other things, such as royalties or the number of author's copies.
In sum, always try to get a bit more from your editors. The worst that can happen is that they'll say no.