Also keep in mind, its still better to do it before hand for other reasons.
If you trespass an officer once he already has his foot in your door and refuses to leave. He will probably make an excuse like "oh he was acting aggressive, or he looked like he was in distress".. but if he is pre-emptively trespassed he commits a crime the second he steps foot on your property before he even gets near the house. So his usualy excuses fall thin. Moreover if he is a n idiot (as many cops are) and tries to use those excuses anyway they wont be valid in court in the pre-trespass case. Since he needs a exigent circumstance **before** stepping on your property, otherwise he can come up with one after the fact.