I live in an area that is classed as rural. We only sound our siren once a month for training and if there is an incident where ANY part of the township is at threat, or if we will need to go near the school during there pickup drop off times the teachers keep the kids in the school and the parents don't mind that either they know we will be coming through soon.
I used to live in area where the siren was used for every call and an all clear but i have not heard this used since I was a kid - personally i think it would excessive.
(I like the siren sets all the local dogs barking and howling - its funny.)