An RO/DI is used to remove impurities in the water source. It doesn't matter wether you use Tap or Well water. What matters is how pure is the source water itself.
If you have TDS meter, you cna measure how much "stuff" is dissolved in the source water. If that number is negligibly low, then you might could get away with not using a filter.
However, I would STRONGLY recommend it.
And, when using well water, you MAY need an RO booster pump (for pressure) depending on how large / strong the bladder is in your stock tank. And understand that an RO/DI rejection rate is something like 9:1 for a good unit (i.e. 9 gallons down the drain for every 1 gallon of pure water) so if your well is low, then you would be wasting a LOT of water....