The thing about the corporation paying less taxes is a myth. The extra contribution you make counts as revenue in their books, and that revenue is then offset as a donation, making no overall difference to their tax benefits.

That said, it does help them in other ways, mostly around marketing. They can then say they've made a massive amount of charitable contributions, when really it was their customers that did so.

As others have said, by making that donation at the checkout, you haven't really made an informed decision about whether the charity is one you would donate to otherwise, so if that's important to you you should stop doing so.

The way I look at it, if you are going to make a conscious decision to donate to charities you support, there's no real reason to round up at the checkout. But if you aren't really going to be donating otherwise and you're not struggling financially, you may as well make that small contribution at the checkout.