My rule is that if you intend to touch the butter/spread/sticky stuff with a utensil, that utensil cannot touch the bread. You just drop the portion on the bread from a height until you think there's enough to cover it, and then you can spread it with that utensil, but if you need to revisit the jar, you need another fresh utensil.
You can't get crumbs in there if there's no cross contamination from the equipment to begin with!
You get better at estimating over time, but having one extra piece of cutlery to wash occasionally is less infuriating than unexpected stale crumbs and food that spoils more quickly from the contaminating yeasts and other organisms.