As a general assumption, given you have no other information on the opponent, then yeah I agree. Obviously your assumptions and "tells" on other players are going to spew over into your schema of what unknown players tend to do. If you have never played poker before and sit down with 7 players and everyone of them is a complete maniac, when another player sits at the table your going to clump him together with the other players, until you get information to prove otherwise.

So since the majority of players at the micros are passive and calling too much, you aren't making a big mistake against an unknown in playing in a manner designed to exploit loose/passive tendencies. However, when you do pick up enough information to alter your schema about that individual, then you should obviously alter your game against that player to best exploit him. For example, to go along with the above example, once you notice the new player isn't playing absolutely erraticly, and instead is a total nit, you should change the manner in which you play against him.