Tire Valve

I Was Wondering If Anyone Knew How Important Tire Valve Stem Caps Were In Maintaining Inflation Pressure In Auto Tires? I Noticed One Of My Tires Losing Air Slowly (from 30 Psi To 21 Psi In About A Week) And This Tire (unlike The Other Three) Also Had A Missing Tire Valve Stem Cap. I've Just Replaced It, And Was Wondering If A Missing Valve Stem Cap Caused Faster Loss Of Tire Pressure.