Cleaning with a damp mop can cause hardwood floors to swell. The general rule is that water and wood do not mix. Wood can act like a sponge when it's close to water, and too much of it will start to swell the boards. Too little, and the boards may shrink as they dry.
Do not wet hardwood floors with a mop. If you can blow the mop or pad, it means you're working with too much water. Excess water will cause damage to the floor and even to the subfloor - something I hadn't considered before. Before continuing with the cleaning, check that the hardwoods are sealed.
If they are, it's OK to scrub a little from time to time. But if not, cleaning with a damp mop can damage floors, as there is no barrier that prevents water from soaking into the wood. Know what you're working with before you start. Flawless basic coatings are designed to clean wooden floors without damaging the finish.
Making sure your customers know how to properly clean a hardwood floor is essential to protecting the integrity of their work in the long term. For long-term maintenance, you should recommend your customers invest in a product such as Bona's hardwood floor cleaner spray. Cleaning hardwood floors with water, or cleaning products with water, is obviously an effective option for owners of hardwood floors. However, it's worth considering if it's worth the risk of trusting a hardwood floor cleaning solution you've found on the Internet.
White vinegar was once a popular product used to clean wooden floors, but it is not recommended for hardwood floors. Dirt and grit are the main enemies of a wooden floor, so before you start cleaning with a damp mop, remove dirt and grit by sweeping with a soft broom or vacuum (don't use a vacuum with a mixer bar).