If you're 'pretty sure' they don't have ulcers, I take it there have been no symptoms of that possibility & no obvious factors they have faced in recent months(or more) that would make it likely? If that's the case, I don't understand why you'd consider giving them meds? Or are there signs & symptoms?
Omeprazole is the usual drug of choice for stomach ulcers. I don't believe it is particularly harmful, if given for short periods of time. It is simply a 'pump inhibitor', preventing the production of stomach acid, to give the ulcers some relief for them to heal. It doesn't actively helps healing, just reduces stomach acid to provide an 'easier' environment. This also only helps stomach ulcers, does absolutely nothing for 'hind gut' ulcers. Because inhibiting stomach acid inhibits digestion of food, it's not the best for long term use.
Esp if you're thinking your horses don't have ulcers, or that they're likely mild, I wouldn't start with drugs, but esp if you're feeding anything hard to digest & potentially damaging, such as cereal grain & other high starch ingredients, I'd quit that & if needed, replace with low starch, healthier options. And I'd include a 'gut support' supp in with their feed. There are 'tailor mades' such as Kelato Gastro Aid, or you can use a herbal blend.
I do agree unless you suspect it that you shouldn’t start with drugs but what has made you think about treating?
Any stressful situation can potentially cause ulcers so I keep mine on a gut supplement called redmond daily gold, I also really like gastroelm and if I suspect an ulcer flare up THEN I’ll treat. As costly as it is to truly treat it’s really not worth doing unless they are prone, vet recommends or you suspect it.
Our barn manager's horse had an ulcer and when they were done with the prescribed medication the vet recommended aloe vera juice be added to his feed. For maintenance he gets Purina Outlast Gastric support--about 1 cup in the morning and evening, with a 1/4 cup of aloe vera juice and some water mixed in to make a mash. Use the juice only though as the aloe skin isn't good for horses.