Which is almost as critical a question as "do you say 'ketchup' or 'catsup'?"
+Les Jenkins tackles the actual underlying question, which is "what practical traditions did you have growing up that you were forced to reassess once you started sharing a household with someone else?" That covers everything from where ketchup gets stored to how you sort laundry to (most important of all) how toilet paper rolls get hung.
For the record, we keep our ketchup in the pantry ("best results" means that it won't discolor as fast, which isn't a problem if you eat a lot of ketchup. We also keep A-1 and Worcester Sauce in the pantry.
But maple syrup (of which we only get the Real Stuff) goes in the fridge.
Butter stays out.
But anything that instructs (vs vaguely advises) stashing in the fridge, we do. It's just that our fridge is usually pretty full, so if we can skip it, we do.
Now, an even more interesting question is what practical household habits you picked up as a kid, that you struggled about with your housemate as an adult, and then eventually discovered that your parents were now doing differently …
(For more on "ketchup vs catsup," see https://en.wikipedia.org/wiki/Ketchup#Terminology, http://mentalfloss.com/article/29649/whats-difference-between-ketchup-and-catsup, http://www.todayifoundout.com/index.php/2015/02/ketchup-catsup/, http://writingexplained.org/catsup-vs-ketchup-difference — general consensus is that in the US, "ketchup" is much more commonly used.)
Fridge.
Fridge. Also, normal ketchup is only for fries and onion rings. Whataburger-brand Spicy Ketchup is a welcome substitute, but still doesn't go on a sammich or hot dog.
Curry ketchup is for Currywurst.
Another question: is it just my adult palate or has Ketchup just turned into sickly-sweet red goo? Didn't it used to taste of vinegar and tomatoes?
Ketchup in the fridge.
Butter in the fridge.
Syrup in the fridge.
HP Sauce (the one true stake sauce) in the fridge.
Worcester sauce in the pantry.
Oils in the pantry.
Vinegars in the pantry.
+Al Hunt Honestly, I don't eat enough of it to tell you. But it wouldn't surprise me.
Fortunately, there are a lot of more "artisanal" ketchups (and recipes to make it at home) that would be more to your liking.
I follow the instructions: refrigerate after opening. (But yes, when you share a home with other people there's tons of these things to negotiate!)