One of tennis' most infuriating moments occurs when after a long hard rally a player makes a clean placement and hears his opponent say, "I'm not sure if it was good or out. Let's play a let." Remember that it is each player's responsibility to call all balls landing on, or aimed at, his side of the net, and if a ball can't be called out with surety, it is good.
When you ask for a replay of a point because you say your opponent's shot was really out but you want to give him or her "a break," you are deluding yourself; you must have had some small shred of doubt and that doubt means the point should be your opponent's. Further, telling our opponent to "take two" is usually not so generous at it might sound.
In doubles, when one partner calls a ball out and the other one calls it good, the doubt that has been established means that the ball must be considered to have been good. The reluctance that some doubles players have to overrule their partners is secondary to the importance of not letting your opponents suffer from a bad call. The tactful way to achieve the desired result is to tell your partner quietly that he has made a mistake and then let him overrule himself. If it comes to a showdown, untactful honesty is preferable to tactful dishonesty.
