My guess is that a tomato is "best" cut by a knife with serrations, and a large roast by a long meat knife, fillet fish with a thin delicate boning knife, etc. Or, you could just chop at everything with a 6" chef's knife and be relatively successful.
When any of my knives can no longer make easy cuts through a ripe tomato without significant downward pressure, that's how I know it's time to sharpen them. You absolutely don't need serrations.
Agreed. Serrated knives tear more than cut. A ripe tomato is a great benchmark for determining whether your knife is sharp or not. Getting back to the thread subject, a single sharp chef's knife can outclass a drawer full of knives of mediocre sharpness.