How appropriate or common is it to recommend referees for your paper?
It's fairly common for journals to ask for reviewer recommendations. This is to help them get reviewers. Suggested reviewers make the editor's job easier, since they no longer need to work that hard to find people who can review the paper.
The downside isn't that it's unethical - I don't see why it would be - but rather that the authors are going to suggest their friends or even commit peer review fraud by nominating themselves. But that's something for the editor to worry about, not you.
Finally it's worth noting that not all reviewers you recommend will be invited to review your article - it's fairly common for journals to have a &quot;one recommended, one self-invited&quot; policy for example. In fact, it's possible none of the reviewers you recommend are invited.