If you are an aggressive player by nature then chances are you will constantly be on the offense. This is the person that will try to go out as soon as possible, with no regard to the tiles in which they discard except that since they are of little use to them, they just want to get rid of them. The defensive player tries to prevent another player from forming melds as often as possible, discarding only the safest possible tiles with no regard to their value in their own rack. Any development they make will strictly be by chance. The ultimate goal of a defensive player is to force every hand to a draw, hoping to luck out from time to time with a fully meldable rack. Obviously, this type of player will win less often than the purely offensive player. However, both players lose out against the good player who knows that there is a time for offensive and a time for defensive, but never plays both all of the time.
Very few racks lend themselves exclusively to either method of play. Even a totally random rack with no reasonable combination can change rapidly with the drawing of only a few tiles. Therefore, it is necessary to constantly revaluate your position as you progress in the hand. If your rack does not develop then you may want to get your point count down as low as possible and play your discards as safely as possible. If that does develop, then you look for useful tiles and probably discard with less regard for the safe value of each tile.
Rummikub differs from Rummy in that all of the tiles that are left on your rack count against you, whether or not they can be melded. If one player goes out, the others cannot lay off on the tiles already down. Therefore, there is a considerable problem for the defensive player or the player who is forced by their rack to play defensively. In deciding whether or not to meld on the table early on in the hand to reduce your point count, or to hold your tiles to prevent them being used by others you should consider some factors in how the hand is shaping up.
You should look at the score and whether or not you are ahead or behind, and whether you want to get the count as low as possible. If you are behind then it is probably best to risk the open melds against holding high tiles. In a defensive position, you should try to hold your point count down to below the spread between you and your nearest opponent if you are ahead in the game.
Then you should look at whether your opponents are playing open melds, or if they are holding them in their racks. If they are doing both then you have to look at specifically what the opponent nearest to you in score is doing. If they play their rack, you can risk holding your tiles longer with the possibility that they will further develop. If everyone else is playing on the table, then you should also play on the table to keep the total score as low as possible for whoever wins the hand. If everyone plays on the rack, you should stay there as long as possible yourself. Either your rack will develop or they will get a high score, which they would have done anyhow. In this case you are working for whatever chances you can find.