Back in the day when the PSR500/Pro106 were the popular models the debate raged as to your present method or all in one was the best method. Below is a good understanding of how the Pro106 works on Multi Sites.
Off: As most trunking scanners have always worked - Pick the first decent control channel and park there until it drops to unusable. (I have reason to believe the actual algorithm is doing a bit more than just this in the PSR500/600 but it's just suspicion for now; the official word is pretty much how I stated it and we'll stick to that).
Roam: The scanner looks at the control channels receivable by it and picks the one wherein the decode rate does not drop below what you set as Threshold Lo; what you set as Threshold Hi determines the "goodness" of the control signal that you have determined it should look for. For example, if you set Threshold Hi to 90% and Threshold Lo to 70% then the scanner will look for a control channel with a decode rate of at least 90% and park on that until it goes below 70% at which time it will attempt to find a better quality control channel.
Stat: The scanner will actually park on each decent quality control channel in succession according to how you have the "Check All CC" setting set. If "Check All CC" is set to "No" then the scanner will park on a different decent quality control channel each time it scans that TSYS. If you have "Check All CC" set to "Yes" then the scanner will park on each decent control channel in the list successively looking for programmed talk groups BEFORE leaving that TSYS and moving on to other TSYS's or conventional channels.
The idea, here, is that you can use Roam in a multi-site system so that, while moving, the scanner will attempt to lock on the best site it can find (according to how you set the thresholds) and stay on that site until the quality drops below what you have set in Threshold Lo at which time it will attempt to find a better site matching your Threshold Hi value, if at all possible. It's not a perfect system and has some limitations that, depending on your area and what other systems share those frequencies, can adversely affect the performance. However, if you move around a lot it may be worth experimenting with it (but see my note below).
Stat is intended to allow you to scan through all decent quality control channels while stationary (say, at home or work) so that you can hear all of the available traffic on all of the available receivable decent quality control channels. This can result in a long scan time, however. It will either take many successive passes through the TSYS to get through all of the control channels (if "Check All CC" is set to "No") and you may miss a lot of traffic on a busy system or it will take a long time scanning through the control channels during each pass through that TSYS (if "Check All CC" is set to "Yes") in which case you may miss activity on your other TSYS's and conventional channels. But it's all a compromise and it's up to you how to use these functions.
NOTE: As I wrote above, the system isn't perfect and does have some limitations. One notable problem with all modes (Off, Stat, or Roam) is that the scanner cannot isolate a system by its System ID, it only uses the decode quality as a filter, therefor, if you have other systems sharing some control frequencies with the ones you want to use within receivable range then the scanner might "falsely" lock on those systems and you may not hear what you wanted to; where I live, it is a major problem as I have many systems outside my desired area with good quality control channel signals that share some frequencies with the system I want to monitor. It most adversely affects Roam and Stat modes, since these involve the scanner most frequently checking other listed control channel frequencies but it can affect the scanner even when set to Multi-Site is set to "Off" due to the rapid changes in signal quality even while stationary (I think it is the many mountains toward my east causing rapid signal fades and phase changes on the strongest site's control channel output).