well my luck is horrible in launching missiles (1 of 5) and so I didn't get to fire a new one, but I went back and looked at the blast radius of a previous one and sure enough there was still rock at the corners of where the missile landed. This confirms your answer.
Correct, no corners are affected by the spherical blast:
^^^^ww^^ww^^ww
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ww^^%%u.UU^.%%^^ww
ww^^%%..%%..%%^^ww
^^^.]ww^^ww^^ww
(oooh, fancy!)