My < $10 purchase was a bottle of black fingernail polish. I used it to paint over the little red light on my bedside cordless phone, recharging bluetooth headset, etc. In bed at night I can't tell the difference when I open or close my eyes.

But on a full moon some light leaks under the blackout blinds. I kinda doubt it makes a difference to my sleep. Same with the other small stuff. It'd be nice to have an idea of the inflection point at which such optimizations don't generate worthwhile returns. But it's cheap enough for me to indulge even if it turns out to be marginally neurotic.