This strategy also has the added benefit of only ticking off one of your opponents, rather than all 3.
When there isn’t a clear target for the robber, one good strategy is to place it on the player whose turn is before yours. Even if that player does have a knight, it will take several turns before they get a chance to use it, as everybody else gets to take their turns first. If that player tries to get in a “robber war” with you, you have the clear advantage, as your turn is directly after theirs and you can player your knights much more quickly. This strategy also has the added benefit of only ticking off one of your opponents, rather than all 3. For related reasons, placing the robber on the player who comes after you should be avoided unless you have a very good reason to do so.