Of course you don't want to overdo this. But used in moderation it's not unethical. And it's certainly effective.
The challenge can be to find good new people to follow, though. Of course you can look through various lists, and that works. But you can often end up following people who are comparatively inactive.
Focus on influencers
With this downside in mind I think it's worth focusing on a different method. Instead of looking through follower and following lists, look at the individual tweets of your favourite influencers. You'll see that many of these have huge engagement, with lots of replies, retweets and favourites.
The retweets in particular are worth looking at closely. The retweeters are listed under the tweet itself. You'll find that many of these people are worth following. That's because they'll share your interest, since they are following the same influencer as you. Also, you know they're active, and engaged. They're also not stingy with the retweets.
When looking at one of these as a potential tweep to follow, check his follower to following ratio first. If it's close to 1:1 then there's a reasonable chance he'll follow you back.
I've been using this method lately and it is the most focused one I've found so far.