Absolutely, there is nothing wrong with wanting to add a preventative. It's just like joint suppliments, just because your horse is 5 and doesn't have joint issues at this point in time, doesn't mean you can't give him/her joint suppliments to add as a preventative.
Why wait till there is a problem? If adding a suppliment to help prevent isn't going to hurt but help, instead of waiting until there is an issue, why not?
My TB gelding does have ulcers, and has digestive issues, but that's because he's 21 and his system is weaker now as opposed to where he was health wise when he was, say 10.
Anyways, I have him on some fabulous SmartPak suppliments, and I couldn't be happier with my choices. He has been on them for over a year now, and looks great, acts great and is going strong *knocks on wood*
I have him on SmartDigest Ultra: SmartDigest Ultra - Horse Digestive Supplements from SmartPak Equine
I also have him on SmartGut Pellets, but that's designed for ulcered stomaches - after you do a full dose of an ulcer treatment.
There is also this product: TractGard - Equine Ulcer & Gastric Health Supplements from SmartPak Equine