Rations and water and stuff like that can generally be overlooked in most D&D situations... Unless of course they're taking a 5 week trip through barren land with nary a tavern in site. Ammunition on the other hand seems a little more important. If you only have 20 arrows at the outset you kinda need to make them last. Ditto with healing potions. I somehow doubt your average quiver holds more than 30 arrows, and even that sounds like a lot to me. Of course you can always retrieve your arrows once the monsters are dead (assuming they didn't break)
This is exactly why I made my
Counters. Makes it a lot easier to keep track of those things for both the player and the DM.