It's just about where you want more ease - leveling or end spike. I tend to rush, blunder and speed through my leveling and then kitchen sink the boss to death (or piety spike him down) and potion spiking is nice there. Not to mention that some beyond end-game strats like Codex-Earthmother are sipplemented nicely by potions because you're powerleveling anyway. Some guys like to take their time with the map and reach the boss with a smooth endgame. And then there's guys who don't even potion spike the boss, because, well, that's just how they roll.
But if you do run into a keg in a subdungeon or something, by all means, go nuts. The level is probably beatable without that keg. If anyone's interested there's a story about why some vets I hold in high regard (and so should you! ) have a different outlook on gnomes as opposed to halflings, it'd make a nice rambling lecture
inb4 rigteous reply from fellow vets about confusing poor newbies: At the beggingin of a kingdom (or playing career) you can't really afford to waste potions because potions is all you've got! So you can't kitchen sink the boss since there's nothing to kitchen sink him with. With gods in the mix, and item unlocks and stuff, you start to rely on gods for you end spike(s) and then potions become viable leveling supplements. So yelling at a noob to conserve potions is actually ethical, right and educational - but gnomes and halflings are a bit of an exception because they let (and often expect) you to use potions as a leveling tool to get an edge.