You don't need to stockpile unless you want to use a lot of them in very short order. After getting the galaxy sword and donating to the museum, Magic Rock Candy and enchanting are the best uses. If you want color changing clothing or color-changing junimos, you'll need to have a few for the sewing machine or putting in the hut, but you shouldn't need too many. You might also want to turn children into doves, but you only need one for this. Otherwise they are a universal loved gift (exception: Haley) and worth 2k each if you need cash. That's it. After getting the galaxy sword, all your prismatic clothing, getting prismatic junimos, enchanting all your tools/weapons... you'll keep getting prismatic shards. If you're finding them, buy some MRC. It is worth it if you're any good in the Skull Cavern.