It can be fun to do, especially working within the NES palette, but it's not strictly required! Up to you. 😜

I've always thought the most possible classic way to go about this is to intentionally misinterpret the cover and do a reverse megaman 1 us boxart situation. As if the boxart for your game only barely hints at what the game is about. But I don't think japan had that problem as much.  But this is one of the many approaches to tackle this so it doesn't matter.