Personally, it seems like a pretty rational decision to me (excluding autopsy problems, which I talk about somewhere else). The reason I advise against it is because I don't believe anyone could possibly know their utility function - and life expectancy - well enough to make a sensible decision about when the right time was to begin the process. This is true even if you exclude the fact that there are good reasons to think that many people do not approach death rationally, and if you consider that an ostentatious decapitation would likely be distressing for those left behind (insofar as you care about the utility of people after your death).

But in a purely hypothetical case - where there was a bomb in my heart that was going to go off in ten seconds and I happened to be standing next to a big vat of cryopreservant - I would highly recommend freezing yourself before dying naturally.