dtilt is good to use when your opponent is in front of you, facing you, and especially if you're expecting a shield grab. the grab will often times miss by going over the top of your low body, and you can IASA another dtilt pretty quickly. Along those same lines, I disagree with cake about dtilt being more punishable. you can use another dtilt VERY quickly after the first, and it's not so punishable (not saying that the utilt is anyway but whatever). Utilt is a good out of shield attack to use when your opponent is close to you or right behind you. I find when shielding a mk's tornado, they often like to try stay right on top of you until the very end at which point they juke you out by going behind you. utilt covers you on both sides pretty well, and i would recommend using it from shield as a punishing attack. It also is great for after a dash attack if they are at low ish percents and can be hit by it right after the dash attack.