This is a great question Camotero and a great use of the forum thank you!
I don't see this as a question of the relative value of truth.
i believe I am willing to go further and affirm that "yes," so long as we are careful with the definition of 'truth," the value of "truth" is relative. Only "pleasure" is entitled to the status of being desirable in and for itself, neither "wisdom" nor "truth" should be viewed in that way, but as always the devil is in the details, or the definitions.
As Pontius Pilate asked, "what is truth?" In your example, rather than "truth," I would say that what you are talking about is "information" -- in this case information about the state of a disease and/or the state (if any) after death). As you say about this information "that good depends on your take of what is pleasurable or no." Certainly the information that the soul is mortal and does not live forever in the happy fields of an eternal hereafter is not "pleasurable" information to hear. It can and does often lead to more pleasure in the lives of many people, who then are freed from fear of hell and armed with the information live life more productively for pleasure. But the information itself is not pleasurable, any more than sitting next to the deathbed of the dying person and reading to them the telephone directory of New York City would be bring them pleasure. Yes the data in the phonebook is "truth," but it is not calculated, relative to the situation, to bring them pleasure.
So yes I would say that if the dying person is afraid of death due to fear of hell, then such a person would benefit from hearing the truth about the absence of an eternal soul. But in your example of a person who has lived their life in the illusion of an afterlife, and is now near death, then such a person would be brought only pain in learning that they have wasted their life and now have no capacity to recover any part of it.
I think many of us face this question in relation to our elderly relatives, and either have or will go through this. I know that I personally have not chosen to use my last hours with them to discuss the eternity of death, while on the other hand I think it is one of the most important issues possible to discuss with younger and healthier people who have the capacity to put the information to good use.
Edit: I realize that I misstated this originally, and this is more correct: "in this case information about the state of a disease and/or the state (if any) after death)." I tend to blend / confuse this question into the issue of "Do you tell someone they are about to die if they don't realize it?"