In south-west Michigan, it is very unusual that a sprinkler control box is hard-wired, unless it is more than 8-10 years old. Today's equipment in use locally is generally low-voltage, with chord and plug wiring for power. The transformer may be chord mounted or in the control box. Though the reasoning presented thus far does not appear the make the issue significant either way, for or against obtaining power from a GFCI outlet or non-GFCI outlet, there is one reason that would lead me to believe that it would not hurt: lightening.

In more than one instance, I have had the unfortunate opportunity to review lightening damage to residences that have occurred when the electrical discharge utilized the sprinkler system to enter the residence and wreak havoc. In one particular case, lightening struck way out in the yard. The current traveled along the piping/control wiring, ripping open the ground as it went. It then entirely decimated the control box, blowing it off the wall and through the opposite wall. Before the "circuit" opened, enough energy entered the residence electrical system to fry most circuitry, and then proceeded to blow the electric meter off of the house. Arching damage was also an issue.

It seems a GFCI outlet might have helped mitigate some of that damage. So, certainly for safety reasons, just keep it simple: plug it into a GFCI protected circuit.