Question: The Saturn V had an initial production run of 15 vehicles (plus 3 test articles). Twelve were used for Apollo 4, 6, and 8-17, one was used for Skylab, the components of the other two are on display in various locations. I've heard there was originally going to be a second production run of the Saturn V, probably including the uprated F-1A engines among other improvements. Was this ever officially considered? If it was, when was it cancelled?

Answer: 
No additional production run was ever ordered, so there was nothing to cancel. The various upgrades considered never went beyond the proposal stage into detailed designs, so there was nothing to order.
What is true is that there were no orders for a second batch of Saturn V Rockets.

There were some attempts to raise budget for further Launchers in the budget plans for 67, 68 and 69. But all were denied by the Congress.

The denial of two further rockets, that should be produced in 1969, by the lawmakers on August 1, 1968 let to the end of the Saturn V production and in the end to the resignation of James Webb as NASA Administrator.

Nevertheless, the Saturn V production line was not closed then and in the integrated program plan (IPP) that Tom Pain and George Muller developed in 1969, the Saturn V played a vital role.

It was only in March 1970, that Tom Pain closed the production definitely, in somewhat that could be described as a "panic" reaction on the 20% cut that the White House made on the 1971 NASA Budget request.