I generally disregard the rarity scale, I tend to use encumbrance rules and try to give the players an opportunity to grab a bag of holding or a heward's handy haversack by third level so they can still carry around stuff without being weighed to the floor. The encumbrance is generally used in my campaigns to determine factors in regards to how they utilize their equipment and loot a place. Anyways, potions I almost completely disregard the rarity scale it's a one off magic item, generally if I give them a really good potion, they are almost certain to not ever use it until one of them can cast that spell anyways because there's always the chance that the next encounter they'll need it and they only have one. I tend to use potions quickly when I play, but the groups I've DMed, not so much. Same goes for scrolls and all other one off magic items, I've also been known to let them find "used wands" that have an unknown number of charges anywhere from 1-5 and only avoid spells that would let them breeze through a major encounter a little too easily. Really fun when they think they have a charge or two left and the wand comes up empty though.