Let me refer you to my mulitboxing thread here:
Also to this official support link here, stating that multiboxing is okay:
I don’t use ISboxer myself, but so long as you don’t break the “one keystroke = one action” rule, and don’t automate anything (i.e. don’t cheat or break the Blizzard Terms of Service) you’ll be just fine. 
I can easily clear all 5 man content in the game, and its a ton of fun!