I understand that if you use the deduction for business use of a home
office that this deduction must be listed as a gain when the house is
sold.  However, I also understand that if you have not utilized this
deduction in 2+ years prior to selling the house that the gain does
not have to be calculated.  What is the actual IRS Rule.  Note: this
is for personal income tax return and the deduction on itemized
deductions.

Any deduction you take prior to the sale will reduce your cost basis
in the home.  If you sell for more than your cost basis the gain is
subject to tax (no 2 year rule, sorry.)  You may be able to use the
"gain on sale of principal residence" exclusion to avoid tax on the
gain and should consult the IRS.gov website for more info.