The low temp salts really shine when it comes to quenching hypereutectoid deep hardening steels (think 52100, o1...)
You quench directly into the salts at shy above Ms (let's say 420 °F) and let just equalize the piece...then you pull it out from the salts and have the chance to straighten it with gloved hands easily. Then let it cool in still air without worry for cracks. It is marquenching, and you get martensite. Instead, if you keep the steel in the salts for long hours you make bainite.
And yes it is the most reliable heat medium for tempering, after the steel has reached room temperature or Mf. The blade will be always at the same temperature regardless its variations in thickness, a nice pro either for carbon and stainless.
For marquenching you need steels that are both deep hardening and hypereutectoids (they have Ms in the proper tempering range); Hypoeutectoids have Ms at too high temperatures to be used for marquenching techinques.
The temperature control will have you free for other activities, but i guess you could stabilize the temp with a simple needle valve in a venturi burner setup. Once the salts are stable in temp you can add the blade and the salts will go down and then up to your setpoint. With a PID when you add the blade the control will start adjusting...overshooting maybe, and bounching a bit. If you already have the salts and tank i'd try to see if you are able to control the temps without PID.