If your life depends on it, by all means use it. In all other circumstances, save the maintenance crew the extra work and the taxpayer the extra costs.
During WW2 there's a good chance they had no idea what they'd be up against whilst they were developing this jet, so they built in a theoretical 2% improvement switch with a large caveat incase the 2% improvement ever gave them the edge.
I think the use of the VMAX switch is only warranted if the ship would otherwise be destroyed or lost if it were not used, which seems like a very unlikely scenario.
- Block IIA (RS-25C): First flown on STS-89, the Block IIA engine was an interim model used whilst certain components of the Block II engine completed development. Changes included a new large throat main combustion chamber (which had originally been recommended by Rocketdyne in 1980), improved low pressure turbopumps and certification for 104.5% RPL to compensate for a 2 seconds (0.020 km/s) reduction in specific impulse (original plans called for the engine to be certified to 106% for heavy International Space Station payloads, but this was not required and would have reduced engine service life). A slightly modified version first flew on STS-96.
- Block II (RS-25D): First flown on STS-104, the Block II upgrade included all of the Block IIA improvements plus a new high pressure fuel turbopump. This model was ground-tested to 111% FPL in the event of a contingency abort, and certified for 109% FPL for use during an intact abort.
It is just a matter of raising a limit. On the mechanical side of things each part has a certain lifetime, you can increase it by reducing strain and vice versa. So they probably already know how much the engines can be pushed if you only need a total lifetime of 1h instead of hundreds or hours. They most likely extrapolated that 2% from this.