If you know your opponent's rating before a match, you have several advantages:
- You can use tactics and techniques that will generally defeat that level;
- If the opponent is lower rated, you can go in with confidence;
- If the opponent is higher rated, you can go in feeling you have nothing to lose, and so play better than normal.
On the other hand:
- The opponent's rating might be inaccurate, and so the advantages mentioned above can all backfire;
- If the opponent is lower rated, you may feel pressure because of a possible upset, or just play down to that level, and lose;
- If the opponent is higher rated, you may feel intimidated, and so not play well.
I would say that far more players have lost matches because of the latter reasons than players who won for the former reasons. In fact, it's not even close. For most players, it's best to approach each match with your own game plan, and worry about the opponent's playing style, not his rating. If you execute properly, you'll beat most of the "weaker" players, and probably lose to most of the "stronger" players. But this is better than pulling off an occasional win by knowing the opponent's rating, and losing five for the same reason.
In most tournaments, an opponent's rating is usually listed on the draw sheet and match slip, and so you usually will know his rating; you'd have to make an extra effort to avoid seeing it. And, of course, you might simply know the player and his rough rating from the past. So what do you do?
Simply put it out of your mind. To reiterate, the opponent's playing style is what is important, not his rating. (Note that playing style includes his level at the various techniques. A rating doesn't give you that info.)
There are some players who do like to know the opponent's rating in advance, for the very reasons given above. As long as you are flexible in your thinking and tactics in the middle of a match situation - and most players are not - that's fine. It's usually better to figure out the opponent's level of play in the match itself, not hope his level matches the number next to his name.
Also, an opponent's overall level isn't nearly as important as what the opponent's level is for each technique. If his rating or level is 2000, but he loops like a 2200 player and blocks like an 1800 player, then your goal isn't to beat a 2000-level player; it's to avoid the 2200 looper, while going after the 1800 blocker.