I painted my entire inside, which seals the wood and makes it a bit less attractive to nesting bugs like mites. All my roosts are also painted. It also makes it a bit easier to clean, stuff doesn't soak into the bare wood. I used a good quality, low vox exterior paint, and it has held up well. It will likely need to be repainted periodically, and roosts get repainted as needed, but I clean daily with straight vinegar on roost and droppings boards, and it holds up well.
I have seen ads for "Chicken Safe paint" Is regular low vox OK. I'm sure the chicken paint is way more expensive.