Almost everyone in our band plays with vibrato when tuning. I wonder if all bands do this. It certainly sounds better, but I wonder if, by doing so, it is actually detrimental to tuning. I know that tuning is more about listening than about adjusting your slides, but wouldn't playing an unaffected pure note give you the best idea of where your tuning is? You'd never use the wammy bar, for instance, when tuning your guitar.