I personally switch between what mode I use very often, depending on what I am doing at any point in time.
If I'm out and moving, I generally want to target more localized traffic so I use the Roam setting to lock onto a site with good decode for the immediate area in hopes that a public safety user is also affiliated to the same site. When that decode drops under the Threshold Lo, then it will drop it and attempt to look for one that is better than your Threshold Hi.
If I'm interested in hearing as much as possibly can, then I might use Stat to go through as many control channels as I can receive in order to hear whatever I can, especially if I'm more stationary. If I want to focus on a trunked system, I might set the Check All CC to Yes. If I want to focus more on conventional traffic, then it will be set to No.
I don't use Off all that often, but it should basically find the first valid control channel and stay with it until the decode drops below it being recognized as a control channel. I believe it should basically follow how you have it programmed into the scanner beginning to end in that order. It might be useful for if you know you want it to follow control channels in an exact order, but I don't use it enough to say for sure.