Personally I do slow intros just to be sure that if there are going to be clashing personalities appropriate steps can be taken to ensure the safety of all. If the current dogs have crates I put them in their crates to create a barrier and to give a bit of time to settle and smell without out having somebody immeadiately in their face. If a crate is not available or I have just 1 dog then I leash them and still make sure it's as slow and controlled as possible. I have never had an issue doing it this way and seems to be a big help anxiety wise.