I do my upper body workout, than throw arms in at the end to beat the crap out of them. Pretty much any motion you will be performing while lifting will work biceps in one way or another, so if you leave curls till end, you'll get a great workout for them. I find MY biceps get way too tired, too fast, without getting a good workout. So when I finish my curls, I cut the weight in half, and lift until failure. People say don't lift till failure, but so far, I've seen no problems with it. I don't over-do it, I just get as close to the line as I can.
And like people have said, don't leave out other muscles. Think about how messed up you would look if you have big biceps and small triceps, or but biceps and a small back... Uhggg, definately weird looking. It's like having 1 arm 2x bigger than your other.