In my experience, trial and error works best. As you probably know, AIRS sites work kind of like cellphone towers where the closest tower is not always the one closest to your phone. It could be one farther away but is providing the best signal at that very moment. For AIRS, it's all about which site a particular radio is affiliating with at the moment the radio is in use. And, just like your cellphone, the AIRS radio is constantly connecting with any site within range for the best signal. A good example of this in action is a Dispatcher transmitting on one site, a fire truck heard on a second site, and a med unit on yet a third. All are on the same talkgroup but computer magic has affiliated those three on three different sites. If you weren't monitoring all three sites, you would miss part of the transmission. Significantly, most AIRS sites are not linked in a true simulcast environment. So, like you have already done, monitor those five sites closest to you and you'll stand a better chance of maximizing your listening.
Now, if your scanner is equipped with an analyze function (like the Uniden SDS100, 200 series), you can utilize it to look at a particular site for signal strength and activity (and several other features) to help determine if monitoring that site is worthwhile.
Shep
