X-acto knife with soldering iron (Durable?)
Given that the blade is made from Steel, and Steel melts at around 1400°C, heating it up to a mere 350°C is not likely to do anything to it other than perhaps tarnish the surface.

Given also that it's not the blade that's "cutting" through the plastic but rather the heat of the soldering iron melting the plastic, it is probably somewhat irrelevant how sharp the blade remains, or whether or not it tarnishes.

I'd be more concerned about the soldering iron. Cutting through plastic leaves a nasty hard to clean residue. While it doesn't really matter if this gets on the blade, if it gets on your soldering iron tip you'll likely never be able to use the tip again for soldering. If you remove the tip and it gets on the heating element then it will degrade performance next time you try to put on a new tip by reducing heat transfer.