This set of posts comes from Pastor Mark Driscoll and his wife Grace. The sermon series The Peasant Princess brought up dozens of questions that they felt they should respond to. The result is this blog series: Christian Sex: Frank Answers to Honest Questions.
Answer: Yes. Providing this act does not replace normal intercourse and does not decrease your oneness, then it may be helpful. Some couples have cited a number of particular reasons why this may be helpful. One husband and his wife do not have intercourse during her menstrual cycle and so she cares for him during that time with a helping hand. Upon occasion, though, she cannot bring him to climax and so he will do so while fondling her breasts so that the two of them are still participating. Some people report that their spouse simply has no idea how to stimulate their genitals and so they stimulate their own genitals in front of their spouse to teach them what they enjoy so that their spouse can then satisfy them. Still other spouses report that during heavy petting and/or deep massage they prefer to stimulate their own genitals while their spouse stimulates other erotic zones and they enjoy the simultaneous sexual stimuli from many places on their body. Refer to Answer #15, then explore freely.