Not true. In Reservoir Dogs almost all the characters use .45s. Mr. Orange, Mr. White (hell, he even uses two to kill the cops in the car), Mr. Pink and Mr. Blonde (though we actually never see him use it in the movie). In Pulp, John Travolta uses a 45., in fact it looks a little too much like the gun Gene Hackman used in Crimson Tide, and after all it is possible that it is the same gun because we know that QT wrote some dialogue for that movie and is friends with Tony Scott.
i also think the gun that Marsellus Wallace uses in the scene where he's trying to catch Butch is a .45
Even Bill in Kill Bill uses a .45 only difference is that it's a colt .45 peacemaker.