Using a reference is not stealing.
Tracing over something is closer to stealing. And while tracing can have its place -- for practice exercises and learning how to perform the strokes with your own media and your own style -- it doesn't generally belong in a finished work, especially for profit. However, textures do, tones do, and sometimes even photographs do; it depends largely on what you're doing with something and the terms of use on the original material. It's a complex issue, and it's hard to draw a line.
I would say, however, that tracing over art that another artist did, especially if it's another comic artist, to put in your own comic...that's probably not going to be okay. That's going to be cribbing, and people will likely get a bit upset over that. Of course, there have been occasions where someone hasn't stolen at all, but they've been accused of it, so it may well be that we're damned whether we do or not.
It's best to have the integrity not to, though. It's one thing to look at a reference, but quite another to simply trace over someone else's work and pass it off as one's own. Even the artists now regarded as great masters used references: models, props, poses, everything they could to make their work the best it could be. There is nothing wrong with using a reference to the fullest extent it can be used. But stealing...
Oh wait, I've taken so long to write this lengthy reply that someone's said it better!
That! And the rest of that post. That's pretty much the best rule of thumb. Don't copy stuff outright and claim it as your own.
