Late Filing Penalty Q

I have a question regarding the late filing penalty -

As background, in previous years I have filed an extension, paid
any amounts I estimated due, usually over-estimating by $10k+. Then
later in the year or next year (after the Oct 15 date) I file, always
historically with a refund due.

In these cases there's no late filing penalty since no amount
wound up being due.

I am still waiting on some data to finish 2006's return (re: income
and expenses for a Schedule C).

I don't excuse the repeated behavior, but it is out of desire to file
the most accurate returns. It has been suggested (multiple times),
though, that filingan estimation then amending the return later when
complete data is available, is the proper way to do it.

So my question is:

If I file by 2006's return by Oct 15 2007 (would be a first in recent
years), but am still estimating, and later (say Feb) have to file an
amended return showing that additional taxes were due, is there a late
filing penalty even though I've filed on time? And if so, from 4/15
or 10/15?

Or, to make it simpler, if I filed 2006's return by 4/15/2007 showing
a refund due of $10,000, then file an amendment 1/1/2008 showing that
2006's return has me owing $10,000 instead, is there any late filing
penalty on the $20,000 delta? Obviously there would be a late
payment penalty.

Thanks very much!

A Late Filer


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