spark plug heat range questions.

the tips of my ngk v-power zfr5f11's had white on the very tips of them when i pulled them out, wich i thought was a little strange. So that means my engine was pingin if there was white on the tips of the insulators? There wasnt any metal that i could see and the white marks on the plugs werent very big, just at the tip of the plugs.
And yes, i have my vaccum advance on ported vaccum right now, i have it set to dd 12 degrees of timing at 18 inches of vaccum.