As noted, you should avoid cross-chaining. It can cause premature wear. And the excessive friction is less efficient.
But under stress it's easy to unintentionally cross-chain. I've done it a few times on fast group rides on roller coaster terrain, forgetting to shift from the big to small chainring, while quickly shifting up and down the cassette. Or, with indexed brifters, I don't want to risk dropping the chain to shift from the big to small chainring on a short, steep, fast, punchy sprint-climb -- could be dangerous for anyone drafting my wheel if I suddenly lose momentum and need to veer off to the roadside to horse the chain back onto the chainring (usually I can do it without stopping, just by shifting and carefully moving the pedals without force). When it's noisy outdoors -- from wind, traffic, other cyclists -- it's almost impossible to hear the telltale warnings of cross-chaining, with slightly increased chain noise.
So I always ensure my chain is long enough to safely shift into the big chainring and largest rear cog. Some folks cut their chains a link or two shorter, but this can damage the rear derailleur, potentially damaging the wheel, frame and injuring the rider. So I'd rather risk a little chain slap from a slightly longer chain. Covering the chainstay on the frame will help protect it from chain slap. Usually there's already tape on the chainstay, but we can add tape (some folks use "helicopter" tape), or even wrap it with handlebar tape, strips from old inner tubes, even twine.
Regarding shifting across the entire range in both chainrings and across the entire freewheel or cassette, it's a matter of setting the travel limit screws to enable full range of shifting without overshifting and pushing the chain into the wheel and damaging the spokes, or getting the chain trapped between the smallest cog and frame cluster where the seat stay, chainstay and dropout meet.
You'll find good tutorial videos for derailleur adjustments on the YouTube channels for Park Tools, Art's Cyclery and a few others. It's easy to do once you've watched a mechanic walk the viewer through the process. Especially with friction shifters since you don't need to worry about the more finicky adjustments needed for index shifting setups.
https://www.youtube.com/watch?v=UkZxPIZ1ngY
https://www.youtube.com/watch?v=wQncKmddahk