Where should you put your putaway shots? Whether they are smashes or loop kills, there are basically three options.
Option One: The easiest spot. This usually would be the longest diagonal. This gives the most table to aim for, and so is the safest and most consistent. The down side - it's also the spot most opponents will expect you to aim for, and so is the most likely to be returned. At the beginning/intermediate level, you should aim most putaways to the safest spot since it's probably not coming back.
Option Two: Aim one way, go the other. Most often this means aiming for the longest diagonal, and then, as the opponent moves to cover that spot, going the other way, usually down the line. This is riskier as you both have less table to go for and you are setting up to go one way, then have to change at the last second, but it's also going to make it very difficult for the opponent to return this shot. It's only at the higher levels that opponents can react and cover both corners.
Option Three: The opponent's middle. This is the transition spot between forehand and backhand. At all levels this is probably the most difficult spot to react to. There are players who can almost relentlessly return shot after shot at the corners, but go at their middle and they fall apart. This is the most common spot top players aim at. The down side - it means you don't get the long diagonal to aim for, plus it's a moving target, depending on where the opponent is. Also, a forehand-oriented player may counter-attack that ball with his forehand - but if he does, he's probably moving early, and leaving his wide forehand open, which is where you would go in this case.