The "Curse of the Ninth" is not a valid reason for concern. As with many such beliefs, it is based upon a number of occurrences which cannot be directly connected the composing of a "ninth symphony". Beethoven is often cited in support of this belief, since he died after composing his ninth symphonic work. However, his death has been attributed to lead poisoning, and could have occurred at any time. Many composers have lived to complete many more than nine symphonies and some have died before this "magical" number. A superstition such as this might be more believable if more composers had died after completing their ninth work and from causes not readily identifiable. Connections made among seemingly unrelated circumstances can be made for any number of details; this does not qualify as "proof". One might as easily find a connection between, for example, the day of the week a set of composers died, making it perhaps, the "Curse of Tuesday" or some other such thing.
Though there have historically been composers who died after composing nine symphonies, there are also many examples of composers who have written more than nine symphonies in their lifetimes. A classic counterexample to the 'Curse of the Ninth' superstition is Mozart, who completed 41 symphonies in his lifetime. The number of composers who died after their ninth symphony is purely coincidental.
The art world is full of superstitions and music is no exception. The "Curse of the Ninth" is the belief that a composer's ninth symphony will be their last. I think that it is just another silly superstition, something that should be taken with a large grain of salt or two.
Like most superstitions, though some composers have met their death upon completion of their ninth symphony, the 'Curse of the Ninth' is in no way statistically relevant. Most superstitious beliefs can find a handful or more anecdotes in support of the particular worry, but it does not give the superstition any more legitimacy. Composers completing their ninth symphonies should not fear death anymore than any other composer.