tach operation thoughts??

i recently had charging issues of which have all been resolved thanks to everyones input. i just noticed something a little strange going on with my tach. i have my tach wired to a switched 12v at the fuse block and no light wired up.the tach is grounded at the dash frame , and the green at the negative side of the coil. heres the thing, when i start the car, the tach reads around 750-800 rpms, which is normal. when i shut the car off the tach drops to around 500. when i turn the key back over into to the "run" position , the tach needle finally drops all the way to 0 rpms. would this be a direct result of a bad ignition switch allowing a draw of voltage through the switch? or could this be another ignition component going bad, or is the tach itself no good? is there an easy way to check? thanks guys for your help. i have pics of the tach when all this is happening, but i didnt think they would be necessary.