If a product is on sale, does the wholesale price stack on top of the sale price?

They do not stack. When the quantity reaches a tier, the tier's Unit price replaces the product's normal or sale price for the whole line. Below the first tier, the shopper pays the usual (sale-aware) price.

If several tiers apply, the one with the highest Min quantity that the order reaches is used. For example, with 10+ at 90 and 50+ at 80, a buyer of 60 pays 80 each.

So when you run a sale, check that your tier prices are still lower than the sale price. Otherwise a bulk buyer could pay more per unit than someone buying one.