Under spark advance, there are a few tables of spark adders (IAC, ECT, EGR, etc.)

Has anyone zeroed these tables out, and what effect did it have?

 

Yes, and it will effect the exact condition you make the change to. IAT - it will add/pull timing if that particular IAT is reached. EGR - same, ECT - same

 

I have not zeroed them, but I have modified the
ECT table to pull -more- timing when the engine gets
too hot (like summertime under load with A/C on)
so that it doesn't ping like a sumbitch with my more
aggressive timing table (dialed in under better
conditions).

And you could use the IAT as a handy "hook" if you
were going to spray it, jack the sensor so it reads
-40 and pull timing in that column.

Zeroing would just make your car ping badly under
some temperature extremes. If you think these
tables are cheating you out of spark, the real
right answer is to fix the cause (fans, thermostat,
cold air intake, etc.) because if the engine were
well cooled and well fed you would never index
into the areas where ECT and IAT pull spark out.