It should be completely fine, the whole point of the thermostat is to make sure that the temp doesn't get too hot. Keeping it around 90 degrees will not melt plastic
^^^^ Exactly ^^^^
Think about it like this, too, the human body is roughly 98 degrees and plastic wont melt in your hand. So even if you have to bump your t-stat to 100 to achieve a 90 hotspot the plastic wont melt.