I'm talking half a teaspoon of sugar in a large batch of sauce, made with around four cans of tomatoes. I'm no dietician but I should think that that amount of sugar would be dwarfed by the natural sugars in the tomatoes. However, it's fine to leave it out. I taste the sauce for acidity before adding it, then add as or if necessary. It's no deal-breaker.

Onion and garlic to taste. I dislike raw onion but we have raw garlic in seafood sauce, a tuna-with-pasta dish in which the pasta is the only cooked ingredient (Well, the tuna comes out of a jar...), and in pesto (added parsimoniously and the only time I ever mince it up). We violently disagree about whether garlic should go into bolognese rágù (I say no, the domestic goddess says yes), and authentic Italian chefs (discuss...) never add it (nor basil nor oregano). So I peel the cloves, give them a quick bash and in they go. They add a pleasant sweetness but not that acrid garlicky hit. All a matter of taste...