I'd say it depends.....
With 7 civvies who are inside only, I see my fair share of eye goop. Some of it is from the dust, some of it is from allergies, some of it is from squabbles with other cats and some of it - oh who the heck knows. I've got cats who never, ever have anything in their eyes wet or dry ever. And then there's the ones where I can clean their face and 5 minutes later they're goopy again. My one zaftig tortie has seasonal allergies that mirror mine and I can pretty much be sure that if I'm sniffling and sneezing then she is too. She often looks like somebody grabbed a slug and rubbed it across her face - and yet multiple trips to the vet over the years have shown there's nothing wrong with her thats worth the cost/side effects of treating. A couple of times a year hers will get so bad that one eye will glue shut and I'll have to take a warm washcloth and sort the problem out for her. Its always on one of the worst allergy days of the year, typically tree pollen. And thats with her being indoor only and me having bad allergies so us always using AC/something with a filter, no open windows here.
On the other hand, one of my others has a reoccuring problem with her eye and requires eye salve every few years. This last time the ulcer on her eye got really bad and the eye swelled shut and she was risking blindness, although luckily she responded well to treatment and is fine. She has scar tissue on the eye, they suspect its from ingrown eyelashes perhaps when she was younger, she's half siamese although I'm not sure if that matters. All I know is she had the problem before I got her and its just been something that shows up at varying intervals.
Generally speaking - if the goop smells bad, is green or yellow, looks like there's pus in it, looks like there's blood in it (as opposed to just a pale red/brown tint), or if the 3rd eyelid is shut, or if the eyelid is swollen shut etc. Well, then you gotta see a vet. If the cat is sneezing and seems to have a cold or infection, you have to see the vet.
But if you just clean a random bit of goop off, and nothing else happens and the cat is not in any sort of obvious distress then you just keep an eye on them and its probably just goop. Its not that hard to get a bit of dust or hair or something in their eyes. I've probably cleaned 4 cats faces today, and at least one of them had that much goop in it (the tortie). Just a normal day here.
