This will be controversial, and there is no right .... or wrong answer. I have friends who will not fly unless there is a First Class seat attached.
While I will not turn down Business or First Class upgrades, when I book trips, I generally book Economy.
Why? Quite simply: the price point. When traveling internationally, a Business Class ticket often will cost more than the entire trip. Personally, I'd rather take the extra $$$ and book another 1 (or 2) trips. Besides, embracing Rule #1, I'm usually there 1-3 days early to unwind. Chalo!