To be fair, a lot of what is written on the internet (as well as a lot of 'common knowledge') is completely wrong.
Solid state devices mostly take a few minutes to reach stable operating conditions once they've been used for a while. Maybe up to ten or even twenty minutes in the extreme. Anything solid state that isn't working ideally after twenty minutes frankly has issues, one way or another. Capacitors ditto. In fact, turning on and off is probably better for capacitors in most circuits. On the other hand, switch-on can produce current and voltage spikes, with inrush current being a notable issue with larger transformers. A sensibly and properly designed power supply will account for this. Sadly, not all power supplies are sensibly and properly designed. Probably the majority of failures do occur at switch-on, although such failures may be relatively simple to repair.
Also, a lot of low-powered equipment relies on 'wallwart' power supplies. Depending on the manufacture, these can have their own problems. I personally avoid leaving any of them permanently powered up if at all possible.
I would never, ever leave any valve circuit on 24/7. Not under any circumstances. Quite apart from the horrendous waste of power and resources and the potential risks, it is likely to seriously reduce valve life. A valve running with no signal passing through it can actually deteriorate faster than one in use.
Even disregarding any 'green' issues, I imagine the majoirity of us are responsible for paying our own electricity bill. Every time that arrives, it's a good reason to start looking round the house at anything left turned on when it isn't being used.
Long and short of it here is that my entire system gets turned off when I'm not listening to it. In fact, pretty much the only things round the house that are left powered are the fridge and freezer and the TV. The last of those somewhat grudgingly, but simply because it has a habit of losing its settings if you power it down for too long.