We learn that Judge Taylor assigned the case to Atticus. Do you think that Atticus would have volunteered for the case, had it not been assigned to him?

Yes, Atticus would have defended Tom. For him, the main part of his decision came down to doing the right thing; it came down to the value of holding to his principles and adhering to the oath he'd taken as a lawyer.

“For a number of reasons,” said Atticus. “The main one is, if I didn’t I couldn’t hold up my head in town, I couldn’t represent this county in the legislature, I couldn’t even tell you or Jem not to do something again.”

"..... every lawyer gets at least one case in his lifetime that affects him personally. This one’s mine, I guess. You might hear some ugly talk about it at school, but do one thing for me if you will: you just hold your head high and keep those fists down. No matter what anybody says to you, don’t you let ‘em get your goat. Try fighting with your head for a change… it’s a good one, even if it does resist learning.”