First, I want to say that in the past, I got many "anonymous" 1-star reviews by users who didn't like that I wouldn't customize something for them for free. I was even told as much by some of them. So, I'd naturally add a 5-star review myself to counteract that kind of crap. My most popular plugin has 19 1-star reviews. Every one of those reviews came before the new system where you must leave an actual review. Requiring that you write something has been a huge plus in cutting back on cheating the system.

I don't see anything unethical about leaving a review of my own plugins. Most of the plugins I build are actually built for myself. I created a solution that works the way I want it to work. Even though I'm the plugin dev, I'm also a user of the plugin.

Doesn't the review/forum system already display "plugin author" next to the plugin author's name anyway?

Regardless of whether you're the plugin author, when leaving a review, you should point out any biases. Not doing so is shady.

It'd also be really cool if WordPress.org could do a "verified install" badge like Amazon's "verified purchase", but I don't think that's going to happen anytime soon.