One of the most frustrating moments that can happen during Words With Friends is when you find a really good play, but are only missing one letter to finish the word. When the word is teasing you with its huge 100+ points yet you can't do anything about it. That's when you need to make the decision whether or not to say, "No I will play you word" or go on knowing of that missed opportunity for the rest of the game.
Even though that was slightly exaggerated it can still be an important move to decide on. There are a couple of certain ways you would be able to try and play the word you want. One in which is where you would keep all the letters you already have for the word and then play the rest of the letters in your rack in hopes of picking up the letter you need after that play.
This is a pretty safe way to go if you are still wanting to get points for your turn. Otherwise, you could simply go the other way by swapping the extra letters if you can not find a place to play them. This means you will lose your turn but there is the option to use the Swap+ powerup so you don't have to.
Nevertheless, there are many things to consider before deciding to wait for a letter to play a high-scoring word. It is good to contemplate which letter it is that you need. Make sure to view the Tile Bag and see how many are available.
If the letter you need is not there as well as any blank tiles, there's nothing you can really do about it. But if there are some blank tiles and letters available, what are the odds of you receiving it? If there's something like only one blank tile it's not very high and would probably not be worth the risk.
Another thing to recognize is the score of the game and how late in the game it is. If the score is really close or the opponent is currently winning it may not be the right way to go. You would have to give up using a significant amount of your tiles if you are waiting to build a word.
This is where some more strategy comes into play. Each turn, try using all of your tiles at first to see if you can make a word that is a higher score than the one you are waiting on playing. If you do find one, there would no longer be any reason to wait since you have found a better option.
If you are still unsure whether or not to try and wait for the letter, inspect the board where you have to play the word. It might not be worth waiting to play a word in its spot if it is in an obvious high-scoring area on the board. The worst thing that could happen is if your opponent plays in the spot that you were waiting for because it had big bonus tiles.
If you are at your last resort, try using a different letter for the word. I once spent two turns trying to get the letter M for the word "volume" but tried using a letter T instead. Sure enough, the word "volute" was valid even though it may not have given me as many points.
So overall, both waiting for a word and forgetting about one are not bad ways to go. It really just comes down to the circumstances on that play and how the game is currently going. There are some strategies that can be applied do make the move less risky just as long as it isn't costing you the game.