The orthodoxy in biology states that every cell in your body shares exactly the same DNA. It's your identity, your indelible fingerprint, and since all the cells in your body have been duplicated from your initial unique stem cell these zillion of offspring cells all maintain your singular DNA sequence. It follows then that when you submit a tissue sample for genetic analysis it doesn't matter where it comes from. Normally technicians grab some from the easily accessible pars of your mouth, but they could just as well take some from your big toe, or your liver, or eyelash and get the same results.
I believe, but cannot prove, that the DNA in your body (and all bodies) varies from part to part. I make this prediction based on what we know about biology, which is that natures abhors uniformity. No where else in nature do we see identity maintained to such exactness. No where else is there such fixity.
I do not expect intra-soma variation to diverge very much. Indeed the genetic variation among individual humans is already relatively mild, among the least of all animals, so the diversity within a human body is unlikely to be greater than among human bodies—although that may be possible. More likely, intra-soma variation will be less than racial diversity but greater than zero.
Biologists already know (even if the public doesn't) that the full sequence of DNA in your cells changes over time as your chromosomes are shorten each time they divide in growth. Because of a bug in the system, DNA is unable to duplicate itself when it gets to the very very tip of its chain, so at each division it winds up a few hundred bases short. This slight reduction after each of the cell's scores of divisions is currently seen as the chief culprit in cell death and thus your own death. But the variation I believe is happening is more fundamental. My guess is that DNA mutates in a population of the cells in your body much as it does in a population of bodies.
The consequences are more than just curious. At the trivial end, if my belief were true, it would matter where you selected to sample your DNA from. And it might also affect when you get it, as this variation could change over time. If true, this variation might have some effect on locating the correct seminal cells for growing replacement organs and tissues.
While I have no evidence for my belief right now, it is a provable assertion. It will be shown to be true or false as soon as we have ubiquitous cheap full-genome sequences at discount mall prices. That is, pretty soon. I believe that once we have a constant reading of our individual full DNA (many times over our lives) we will have no end of surprises. I would not be surprised to discover that pet owners accumulate some tiny fragments of their pet's DNA,which has somehow been laterally transferred via viruses to their own cellular DNA. Or that diary farmers amass noticeable fragments of bovine DNA. Or that the DNA in our limbs somehow drift genetically in a "limby" way, distinct from the variation in the cells in our nervous systems.
But I consider all this minor compared to a possible major breakthrough in understanding. We have a pretty good idea of how the "selection" in natural selection works: less fit organisms die. But when it comes to understanding how variation arises in Darwinian evolution all we can say is "random mutation" which is another way of saying "we don't know exactly." If there were intra-somatic variation and if we could easily observe it via massive constant full-genome sequencing then we might be able to figure out exactly how a mutation occurs, and whether there are patterns to those mutations, and to what extant such variation is induced or influenced by the body or the environment—all ideas which currently challenge the Darwinian wisdom that the body does not directly influence the genetic makeup of a cell. Monitoring genetic drift within a body may be a window into the origins of mutation itself.
Even if these larger ideas don't pan out, the simple fact that DNA in each cell of your body is not 100% identical would be worth investigating. Such a fact would be a surprise, except to me.