Are water softeners safe? What about garbage disposals?

Both are safe as long as the rest of your septic system is functioning properly.  They do both add an additional strain on your system, so you should have your tank pumped every two years to help maintain your system.  Also, much of the stuff you put down the garbage disposal will not break down in your septic tank, so try not to use it aggressively.  Coffee grounds should never be put through the disposal.