Yes, astronauts that are out of orbit should be granted tax extensions. When an astronaut is out of orbit, he is in service to science and society. As such, he or she should be given certain exemptions in all financial arenas. Considering the risk to life and limb involved this is the very least we can offer these brave explorers.
Astronauts certainly should be granted a tax extension when they are out of orbit. Many long term space missions mean that the astronaut will not be able to meet deadlines needed to file their taxes, it would only be fair when they are physically unable to file that they are given some leeway.
On one hand, the case of an astronaut who is in outer space when tax day comes around is such a rarity that it would seem odd if the IRS had an official policy on the issue. But now that the issue has come up, it seems obviously very reasonable that an extension should be granted. Anyone who has passed through the extremely rigorously process of becoming an astronaut is obviously very responsible, and can be counted on to take care of their taxes upon their return to earth.
Astronauts in outer space should absolutely be granted tax extensions. They are off-planet charting a course for humanity to move into the 22nd century and onwards. If they can't get back to terrestrial soil on account of being fearless, forward-thinking explorers in time for tax day, let them slide. Their work is our only hope for future as a race. Tax day is just an earth deadline.