In 4th ed, the Raise Dead ritual specifies that "you must have a part of the corpse of a creature." Is a part removed from a living body considered a part of the corpse once the original body has died? Or does it have to be a part removed once the person is a corpse?
For what it's worth I've already made a ruling in my own game. That ruling is "maybe." Because it's a tenuous use of the spell it sometimes works and sometimes doesn't. In the case of a removed finger providing resurrection insurance, I'm going with the caster making a religion check. That value is the percentage of a chance that it'll work. This rewards higher level casters (which is important because it means the players will always be seeking out a better insurance policy), but it's never a sure thing.
Best Answer
There are two alternative interpretations to answering this question, both of which are very important.
First, and critically, players doing this are signalling that they don't want to stay dead. Turning around and saying "No!" means that you're negating a player wish. Certainly allow them to specify what level of body part is needed for resurrection. Toss the question onto the players, especially if one of them is a wizard, and try to this in character. Then, inform them that the entire world will be held to the same rules. Let them decide the consequences and hardships they want, don't impose arbitrary hardships on them.
Fun, and more to the point, antagonisism, don't come from negation. Allow the players to be crafty, reward them for having excellent ideas, and then insure that they're aware of the ultimate consequences of their actions. This means that they've had an impact on the game world, instead of a fun-destroying No!. Never say no. It's boring.
Secondly, looking at the problem in the 4e world from an Ars Magica perspective, fingernails and hair can certainly be used as Arcane Connections to a body. Perhaps, taking a page from that, the 4e DM could request that such ephemera be "enchanted" to form a "lasting connection" to the person. Of course warn them that many rituals can use such a lasting connection. The law of sympathy, from actual magic theory certainly holds that a fingernail is as good as a person. Let it be so in 4e. Let there be Magic!
Then let there be complications! Not all the time, cause that too is boring. But it's details like these which inform the game world, and give the choices of the players weight. Dying is boring, or exciting. Let the players decide. All of their decisions have interesting corruptions and consequences, so there's no need to restrict the design space!