2012 Mazda 3
I use cruise at every opportunity. I leave the system "on" permanently, then engage the accele/resume/cancel buttons as needed. Does leaving the system in the "ready" mode like this cause any harm?
This should be safe logically and based on personal experience. Most cruise control systems use engine vacuum to pull on the throttle cable and the vacuum is always there when the engine is on anyways, so there is nothing mechanical running behind your system switch.
2012 Mazda 3's have either electronic throttles or cable. The cruise control system with an electronic throttle is pure software and just uses the same throttle control hardware that is used when you mess with the accelerator. So you should be good to go either way.
I have left my system switch on for the last 9 years in my civic, and passed 312k miles this way with no problems to my cruise control system; or any electrical system problems for that matter.