Supercapacitor voltage rating vs supply
Typically a rule of thumb for low voltage capacitors is 20% tolerance minimum. That being said, if this is a non inductive circuit for example the output from a 5V LDO powering a low power embedded system and the capacitor has a surge voltage rating of 6V, I would say it would be alright.

Also note: the datasheet lists "5V" as a feature, this could point to them expecting use in 5V circuits.

Just make sure you are not coming close to any other specs such as current draw, temperature range etc that could cause additional stress.

Also take note of specifications like leakage current, my guess is at its maximum voltage this will start creeping up a lot.