Would a diode be needed on coils 85 and 86 for all relays used such as relays for multiple sirens or the horn honk on alarms? OR are they only used for kill relays?

 

The diode that is installed across #85 and #86 on a relay is to prevent arcing at the high current contacts, #30/#87a/#87 on something like a starter kill relay where current draw is relatively high a diode should always be used, you can easily get away without one on something like a second siren although it can't hurt to install a diode on relays that are switching relatively low current. 94