If your travel plans aren't flexible, then buying early is usually the best deal. Looking at flight status of the same flight over the next days (like, 3 days out) can give you an idea of how full they get -- if they're basically full a few days out, your chances of upgrading might be quite small.
If you do waitlist, then the fare class does make a (small) difference. Buying a W fare you often won't get the upgrade (especially if you fly through SFO). However, even with the higher fare you still may not get it if it's heavily booked, or there are global service members who are waitlisted. So, it could be more money you're throwing out the window (already you're throwing money out the window by booking a W fare!)
The "upgrade sniping" described in the thread happens, and I've lost a few waitlisted upgrades to snipers, but it is not the norm. Usually, waitlists do clear as they're supposed to.