Another phrasing for "playing on the psychology of numbers" would be "Using language that makes the customer more comfortable." Is it evil psychology to say "please" and "thank you" to people walking into your shop (also will likely increase sales)? Or evil psychology to learn to use appropriate metaphors when giving a speech? Is it evil to use a nice font rather than comic sans?

Because people are provably more comfortable buying low value items with .99 at the end. You make them uncomfortable with the purchase by rounding up, and either way, you are going to be rationally choosing one of the options.

On the other side with your logic, by pricing as a whole number, you are using evil psychology to persuade your customer that what you're selling is a higher quality and more expensive item. So evil!