Let's say a Corporation with a calendar year Fiscal year and Plan year does not go on extension and files their 2006 tax return by 3/15/07. If they prepare an amended 2006 tax return filing prior to 9/15/07, can they increase the profit sharing deduction that they had taken on the original filing as long as they contribute this increased amount by 9/15/07. Is this allowable/deductible/possible/plausible....?

no. you can only increase contribution if you had filed extension by 3/15. Cannot amend tax return to increase contribution. See REv Rul 76-28.

Thank you.

So that seems to be a reason that corps should always file an extension.
A lot of clients seem to always want to file by 3/15. Is there a reason for this other than
perhaps their CPA's charge for preparing the extension.
I guess I am curious as to why some want to file by 3/15 each year.

Reasons:

1. Fear (probably misguided) of increased risk of being selected for examination.

2. Desire not to extend the period of limitations for assessing tax deficiencies.

3. If it is an S corporation, in order to get K-1s to shareholders before April 15.

OK.

But even if you file by 3/15, is there any reason why you can't (or shouldn't) also file
for an extension as the mere existence of an extension form seems to keep open some
options for you?

there is an IRA ruling that allows a corporation that files both an extension and the tax return on the due date for the return to defer making the contribution until the last day of the extension.