I read in classic fishing manual (LUNKERS LOVE NIGHTCRAWLERS) many years ago to always use the 'heaviest line you can' and still catch lots of fish.

That still applies, as far as I can tell, to trout fishing with a fly rod.

Things have changed alot regarding tippet nomenclature versus breaking strength. I have an old spool of Scientific Angler '5X' tippet that lists the breaking strength at 2 pounds. The 'new' 5X stuff I have breaks over 4 pounds now. THAT is a significant increase. Also, the advent of flourocarbon with it's invisibility factor plus an even stronger strength to diameter ratio means that you can use much stronger line than our fly fishing predecessors could and still get the fish to eat the fly.

I've always used 5X as the default tippet strength when trout fishing. Over four pounds of breaking strength is plenty for trout up to around 30 inches in average waters. Adding in obstructions, large flies, or heavy current will require a bit heavier, finicky fish, tiny flies, or very clear waters might need finer stuff.

It's always a balancing act. The key for me is light enough to not spook the fish....I'd just as soon hook them and lose them as not hook them at all.

Buddy