With respect to OP, there are many "artistic" shots with extended exposures. Unless you have a strong light source (sun, focused light aka lasers) shining onto your sensor or are pointing towards one... should be okay.
i have taken countless 1-8 minute shots with my k20d, it is not a d90, but i don't see how that would make the camera spoil easily, unless you do something silly, as mentioned by others, like 5 minute exposure with the sun blazing high in the sky.