It's really difficult to determine whether a pin is 'valuable' because value is so subjective ... and there's also sentimental value and monetary/trade value. Pins with a high sentimental value, well, those are important to because they *mean* something to you and you wouldn't part with those for the world no matter what they're objectively worth. When it comes to monetary/trade value, at the end of the day, a pin only is worth what someone is willing to cough up, be it $$$$ or pins, to acquire it.
That being said ... how important is this pin to you? Do you think you'd want to keep it or trade it? If you want to keep it, then it really doesn't matter if you wear it on your costume (do NOT wear it on your lanyard ... anything worn on your lanyard is for trade; as I understand it, award pins to be worn on your costume) - but if you choose to wear it, I would not use the rubber Mickey back as those are likely to fall off and you could lose the pin. Just know that wearing the pin will not keep it in mint condition - but that's fine as many people wear pins they don't intend to trade or sell. If you want to keep it safe, you also could do that. At the end of the day, you need to do whatever will make you happy, because *that* is what pins should be about, not 'value.'