It makes no sense. You play who you think will put up the most fantasy points. Period.
For example, let's say you are debating between starting Plaxio Burress and Donald Driver. Ignoring who you are facing, let's say you project Plaxico to get you 10 points this week and Donald to get you 12 points. But wait, you're opponent is starting Eli Manning. Should I then start Plaxico? The answer is a definite "no". Eli Manning will still put up the same amount of points whether or not you start Plaxico or Donald. The object of the game is to put up enough points to beat your opponent and the way to do that is to start the players who you think will get you the most points REGARDLESS who you're opponent is starting.
Ok, I'll put it this way so you'll understand.Let's say you project that Plaxico gets you 10 points this week and Donald gets you 12 points and compare boom and bust days for Eli for each player.
Eli bust:
Eli (14 points) - 200 yards, 1 TD (4 points/TD, 1/20 yards passing)
Burress (5 points) - 50 yards, 0 TD
Eli boom:
Eli (27 points) - 300 yards, 3 TD's (4 points/TD, 1/20 yards passing)
Burress (27 points) - 150 yards, 2 TD's
In the bust scenario, the difference between starting Driver and Burress would be 5 points (14-10 vs. 14-5) in favor of starting Driver. However in the boom scenario, he difference between starting Driver and Burress would be 12 points (27-10 vs. 27-27) in favor of starting Burress.
Starting Burress is insurance in case Eli blows up. Of course there are other variables to consider, such as how much of your opposing QB's targets go to your receiver and if the QB has the ability to put up big numbers. In general I think it's a smart move to counter your opposing QB with his #1 receiver if your WR options are similar in projections, especially in leagues where QB TD's are 4 points.