Oh, that's a huge question. I could write a page and only scratch the subject.
At idle stochiometric ratio is ok, i.e. 14.7:1. For cruise leaner will give you better mileage, but a CVH will start to bog and lose efficiency after 17.0:1. At low boost (~0-10psi) and at off boost WOT a 12-13:1 ratio will be ok. At higher boost you would want to run more fuel than what is optimum for power for safety against melting and for detonation prevention. Anything richer than 10.5:1 and power will go down rapidly after that. It' sprobably needless to say that many parameters affect the wisest ratios like IC, C/R, turbo size, cams, combustion chamber, etc.

Whenever you loft the pedal with revs over ~1800rpm you activate over-run fuel cut. You are essentially braking with the engine and it would be pointless to inject fuel. When the revs drop low enough fuelling is resumed not to stall the engine when clutch is pressed.