There is nothing unethical about standby. I do it all the time for the purposes of saving money. The airlines allow it as part of the benefit of buying a ticket. If they felt standby was "unethical", they simply wouldn't allow it at all. (Southwest, in a way, has done this. Other airlines tried to take a harder stance about a year ago, and then relaxed.) So we don't even need to have an ethics discussion: the standby process is built into the rules and policies themselves.

What we are talking about here is purely risk. When you buy a late-night flight and standby earlier, your risk is the cost of your time spent standing by in the event you don't get on the earlier flight. (You know you haven't risked your original seat.) When you buy an early flight and standby later, your risk is that you lose the entire itinerary plus at least some value of your ticket. It's up to you if you want to take that risk, which most of us would agree is higher than the more common standby approach.

In short, if it were a dirt-cheap LGA-DCA ticket with 15 flights a day, I'd probably risk it if the price was right. If it a transcon with 2 flights a day, I probably wouldn't risk it at all. But that's just me, and your risk tolerances will undoubtedly vary.