As is the case with almost anything in life, what works for some isn't always the best for others.
My local county Motorola system - which is about at the end of it life and in the planning stages for replacement - and is a countywide, single simulcast system that is mixed mode (about half analog and half P25/digital voice) regularly suffers from some type of issue that results in the "rolling" of the control channel between the primary and one or more "alternate" but known frequencies.
Anything is possible but most of these types of systems "broadcast" to subscriber radios what frequencies the radio should be aware of that might end up being a control channel (for the radio to check) should the signal on the "current" control channel disappear.
In most cases, these primary and alternate CC frequencies are posted on the RRDB. However, this information is RR user provided and subject to incorrect and/or incomplete information and changes that aren't updated, etc.
So, on the surface, it might make sense to enter ALL of the frequencies in your programming. That too could still result in losing reception should that list be incomplete and/or undergo changes without someone reporting those updates in a RRDB submission.
Having said that, there are other cases where programming everything isn't a good choice -- mostly, those cases being newer systems that exhibit "digital simulcast" issues. While this may not apply for the older Type II systems, it is still something to consider.
Keep in mind that if your scanner loses sync/track of the control channel, it will then start looking through all of the frequencies you have programmed for any given site trying to find where the CC went. In newer systems, the scanner typically just lost sync and starts searching only the find that the CC didn't change. The fewer frequencies for the radio to check, the faster (potentially) the scanner reacquires the CC.
For my state P25 system, the CC frequency is pretty stable so I tend to only program the primary CC frequency for each site for the best performance.
In most cases, I also treat each system "site" (and associated control channel frequencies for each) as a separate "system". This way, my radio doesn't make decisions for me which site it thinks I should/want to be listening to.
Again - every situation is different and the power is in your hands to program what works best for you - once you understand how they may or may not work. In life there is "text book" but then there is reality that many times is not the same.
BTW - with reference to "only to enter the red control channel and the scanner will populate the rest" - what was meant here is that the scanner will read data from the control channel (but not for all system types!) and determine all of the voice frequencies from that data. However, the scanner do not "populate" your scanner programming - it just uses that data "on the fly".
