Reasonable people may disagree about what counts as unethical, but I don't see any gray in this scenario: if you're convinced that something is bad enough that you need written cover from a superior before you'll do it then you just shouldn't do it. Likewise if an employee comes to you and says "I want to do this thing but it seems smelly so I want written approval from you" you shouldn't approve it.

There are things you shouldn't do because they're plainly unethical.

There are also things that are ethically fine, but maybe your boss' boss, or some other department, or the customer, might not like. In those cases, it can be a good idea to document that you didn't come up with this on your own.

Stuff that requires a SOx audit trail (or equivalent local, or even internal, equivalent) is definitely not OK to do if asked verbally but (probably) fine to do with the correct paper-trail in place.

And I've definitely used "please put that in a ticket" as a stalling mechanism so I can go away and check that what's being asked for is legit.

But whether or not you should create fake reservations to harm your potential clients so they give you money is pretty damn simple. Sure you can find complex ethical dilemmas, but most issues at work are either right, wrong, or unclear. Getting stuff in writing helps keep honest people honest, and can help make things clearer. I think GP's point is that getting it in writing doesn't move it from wrong to right.