I've never owned a leather instroke case, but have owned enough leather products that I would never "just leave it alone". Untreated leather will dry out over time. Dry leather will eventually get brittle. Brittle leather will crack when it bends. That might not happen for years, depending on the climate it's kept in, but it will eventually happen.
Sure, oils and waxes will darken the leather a bit (in my opinion that's usually a good thing, gives it some character), but it's well worth it.
And if you don't go overboard with the products it should only be a slight difference.
For my razor strops I use 100% pure neatsfoot oil. Does an amazing job of keeping the leather moisturized without leaving unwanted residue on it...but they never leave the house and are stored and used very carefully.
For something like a cue case, that may see the occasional rain or spills, I'd look for something with a sealant in it.
A "Neatsfoot Compound" would have the benefits of the neatsfoot oil, plus some waxy stuff that will make it slightly water resistant.
There's also things like mink oil, which I've used in the past for a great waterproof barrier. Or spray on stuff that you can get in leather stores or shoe stores, I find it doesn't work quite as well, but is better for suede or rawhide because the oils or waxes gunk up the rough surface.
So, what you choose to use will depend on what type of leather the case is made of, but I would definitely suggest using something. I'd say moisturizing the leather is mandatory, waterproofing it is preferable.
If it's treated well, it won't be the end of the world even if something does get spilled on it either.
If "left alone" and the leather is on the dry side, it will soak up as much of whatever is spilled on it as it can. Consider dry leather to be like a dry sponge, it wants to wick up moisture.
If it has waterproofing treatment, just wipe it off right away with a damp towel before it dries on it.
If it hasn't been waterproofed, but is well oiled, the oil will stop most of the spilled liquid from soaking in. Consider oiled leather to be like a saturated sponge...might be able to hold a little more, but certainly isn't going to soak up the whole mess.
If a spill does leave a mark though, try cleaning with saddle soap and re-apply your conditioning product