Do lawn tractors and lawn mowers need new spark plugs every year to insure peak performance ?
How do you define "peak"?
It kind of depends on each individual engine as to what works best cost wise.
On my 24? year old Honda walk behind mower, I change the plug every 5-6 years. I "think" it seems to help, but it'sprobably more of a placebo effect.
Previous mower with a Tecumseh (Craftsman) engine I would replace about every 15-20 hours or it would become hard starting when hot. In retrospect, it might have been a heat sensitive ignition coil??
I'd go for hours used and then pay attention if it REALLY makes a difference or not. Adjust hours accordingly.