There's not going to be anything really governing 'serving hard' as ethical or not ethical. I played volleyball for 6 years, so I eventually had a pretty good serve as well. In PE class, or at parties (4th of July, etc.) that tend to have a volleyball net up, I would rather do Float Serves instead of just a power serve/ jump serve. It's less cocky looking, and when done right can really mess with your opponent a lot more than a brute force serve -- think of what it looks like when a knuckleball pitcher fools the batter. Not many people in PE classes or Parties have played volleyball a lot, but rarely you can identify your "rival" (someone else who plays volleyball :P), and you and that person can set up the teams so you serve to each other.
Plus, people who don't know how to properly bump the ball will start to get really painful forearms: redness and sore arms - which is kind of mean.
Of course, underhand serve, or overhand bloop serve it to people who either A. have never played before, or barely played, B. Are younger:
- Have never played before, or played very little
- Younger players
If you are playing against anyone experienced, there is never anything wrong with serving it hard at them, they should expect it.
Long answer, short:
Nothing wrong with serving hard to experienced players, if they are new or young, serve lighter as a good sport.