If you had a choice, you should always prefer to be raising. There are so many advantages to raising: you may win the pot there and then; you may drive out threatening or even better hands than your own. You may be able to bluff successfully even if the flop misses your hand completely. It sets you on the high-ground – from below players will find it hard to attack you.
With the premium hands: AA KK QQ and AK, you should always raise, even if a player has raised before you. There are many players who prefer to slow-play strong hands. This means that they pretend that they have nothing until the end and then attack aggressively, hoping to lure in weaker hands. Such play is highly risky, very stressful, and extremely hard to judge well. Whilst it does, without doubt, produce some fabulous results sometimes, it also produces disasters in which players can lose a fortune.
Generally, with strong hands like those above, you should protect them by raising and driving out players with more speculative hands which might suddenly overtake you once there are three, four or five community cards with which to work. If no one calls, then the chances are that you would not have made much from the pot anyway. If you get callers, you may be set for some real action – with the odds on your side.
In the final analysis, you will decide what style of poker player you are. You may find that the other players at your table are very respectful of raises and, when you do raise, they mostly fold and concede the pot to you. If that is the case, you might even try raising modestly with all of the 21 chosen hands and seeing what effect that might have on your table… Of course, you may discover that you are surrounded by chancers, players who will pay right up to the end to see your cards. These punters are sometimes called "fish" or "fishes": they swim around "fishing" for the perfect card to make their hand a winner. Fish are usually slimy characters, poor, near-penniless souls who have failed to realize that, most of the time at the poker table, discretion really is the better part of valour – and that it is certainly cheaper in the long run.
So, how you act will have as much to do with the style of the other players at your table as the cards you hold in your hand.