As you already know, it is best to keep your melds for the last possible moment, but there are going to be times when you simply can’t keep them in your rack any longer. As the hand progresses you may see that this is the case and you are going to need to change your strategy. It may become desirable to begin playing the open melds, and you have to be prepared for this.
If one or more of your opponents is down to three tiles, the chances are that they are looking for one or two possibilities to complete their hand, and you do not want to be caught with a large point count. There may also be times during the hand when open melds appear that allow you to play odd tiles on your rack which you do not see much chance of combining. Therefore you should play early on in the hand, to form melds with enough points to allow an initial meld. During the play you may wish to rearrange these on your rack, but you should always try to maintain the necessary 25 points to make an initial meld. If you should be lucky enough to have a joker, it is always possible to meld it with two other tiles for a guaranteed initial meld, but that may not happen, not to mention that it isn’t wise in general to meld with a joker to begin with.
Again, you should try not to make an initial meld until it is to your overall advantage, either defensively or offensively. If you have reached a point that you feel that it is necessary though you need to remember that you do not have to put all of your possible melds down. The rule is still to play as few of your tiles on the table as possible. By keeping some tiles behind you are making it harder for your opponents to use your open melds and possibly going out, gaining more points.
There are going to be times in which you have tiles on your rack that you are certain that your opponent can use to their advantage. This is when you most definitely hold back these exact tiles. The last thing you want to do is to give them an advantage over you. When you hold tiles defensively they usually have no offensive value to you, so therefore you must attempt to build possible combinations around these tiles that will allow you to meld them later on. Sometimes these defensive tiles can be played to allow your opponent to use them and give you an advantage on your next turn, but you need to make sure that you can use them the next time around.
It may seem reasonable to assume that it is always to your advantage to have an opponent play to the open melds while you retain your tiles and form your melds on your racks. This is generally true when you are dealing with an average rack. The trade off which you must consider is that any tile on the table is of no value to anyone as a score. The only scoring points you receive come from the tiles that remain on opponent’s racks after you go out.