The TS type irons and the T12's are however 100% not safe devices. The tip mount/socket shorts out which will blow the guts out of the MOSFET and there are various brown-out conditions which cause the thing to crash and dump the entire supply into the tip until it glows orange. And also the thermal sensor fails unsafe. That risk is especially high when you have high capacity supplies.
An image to back up my claim:
Buying a decent, simple iron will probably cost you less than your house does if it goes up.
As for leaving irons on I have never done it. Metcal PS900 also has an optional stand which puts the iron in safe standby mode. They heat up within 2 seconds when you pick the iron up as they do not use normal heating elements. They use RF heating. There is no condition that can cause a complete thermal overload cycle.
On the T12 irons which are sometimes mentioned they are borderline illegal when it comes to the power supply design earthing and will quite happily kill you dead if you don't rework them first.
Is economy worth it? Probably not.