It occurs to me that the author has the bit of dialogue in which Sergeant-Major Morris tells Mr. White to wish for something sensible because it is essential to the way W. W. Jacobs wants to develop his plot. Jacobs wants to make it questionable whether the wishes are granted by the monkey's paw or whether they are the results of pure coincidences. The author has Mr. White tell his wife, for the reader's benefit:
"Morris said the things happened so naturally," said his father, "that you might if you so wished attribute it to coincidence."
So W. W. Jacobs has Morris warn White to "wish for something sensible" because the author wants White to make a modest wish and not do what many of us might do if we had the paw, which would be to wish for something extravagant such as a million pounds. It could be a pure coincidence that Herbert's employers pay the parents two hundred pounds compensation for the loss of their son, but any fantastic amount would be out of the question and spoil the story.
Sergeant-Major Morris's advice to "wish for something sensible" foreshadows the arrival of the representative from Herbert's employers to inform them of their son's death and to tell them that the company wishes to pay a compensation of two hundred pounds. If Mr. White had not wished for "something sensible," that is, for something modest, the night before, then the compensation could not seem like a mere coincidence. And W. W Jacobs, the author, wanted the results of all three wishes to seem to as if they could have been mere coincidences.
In W. W. Jacob's short story "The Monkey's Paw," Sergeant-Major Morri's story of the paw indicates that the paw interprets wish requests very literally, without any breaches in logic or of the space-time continuum, breaches people often see when picturing their wishes fulfilled since they fail to draw full, logical conclusions about how their request will be fulfilled or of the consequences of fulfillment. Therefore, if one is to make a wish, one needs to be careful that the request is logical and literally fits with what one wants, which is exactly what Morris warns of when he says, "If you must wish ... wish for something sensible," rather than for Mrs. White literally having four hands, which would make her a freak of nature. Morris's warning to "wish for something sensible" foreshadows the paw quite literally interpreting the Whites' wishes and turning them into something horrific they don't really want.
One clue that the paw interprets wishes very literally is seen in Morris's statement that his predecessor wished "for death," which is how the paw came to Morris. Therefore, the reader senses that Mr. White's wish for "two hundred pounds" will bring dire consequences because he failed to specify by which means he wants to acquire the two hundred pounds. Due to the earlier foreshadowing, the reader can instantly sense that a wish for just "two hundred pounds" without any clarification will bring the Whites something they don't truly want. Consequently, Mr. White acquires the money as compensation for his own son's grizzly death. In addition, Mrs. White's request to have her son alive again, without specifying a wish that the tragic accident had never occurred, only serves to bring her son home in his mutilated state, since, again, she made a wish that was not sensible by failing to grasp the true logical consequences of her exact request.