Reading from & writing to a texture at once? (FBO)

I have a pretty complex set of deferred rendering steps. Is it okay to bind a texture and read from it in a shader, while writing to the same texture, which is bound to an FBO? It seems to work on my GEForce 8, but am I breaking any rules doing this?

From the spec :

(44) What should happen if a texture that is currently bound to the
context is also used as an image attached to the
currently bound framebuffer? In other words, what happens if a
texture is used as both a source for texturing and a
destination for rendering?

        RESOLUTION: resolved, (b2) - results are undefined because
        the framebuffer is not "framebuffer complete".

Even if it works with your card & your current drivers, depending on this fact seems a bad idea.