A technician told me once that using the devices plugged in all the time (when in use) should be varied once in awhile, letting the battery run down to the notification to charge. Also, when things are plugged in when OFF, there is still current passing, right? I do NOT leave it plugged in when OFF.

Unless you use the device for long periods at a time. Otherwise the battery could be drained down to almost flat, if you Only use it for short periods at a time, giving no chance for a full recharge. (Nothing is 100% efficient)
I don't know about devices now, but there used to be devices that did not charge when they were in use. They charged only when they were off and plugged in.

A tech told me that leaving the wifi ON while the thing is OFF would cause it to detect signals from the router and keep trying to connect, therefore spending the battery.

As for "devices that did not charge while in use," this one is charging whenever plugged in.

That On/Off switch usually isn't. It is just a momentary push button that controls (toggles) the state of a simple logic circuit.
Shutting off Blue Tooth or the WiFi helps, but even then parts of the circuit will still be working.
On the crime show, ever notice whomever removing the battery from their cell phone, or someone suggesting doing so, so they couldn't be tracked? That's why. Most cell phones can be turned on remotely. Those circuits always draw power.

than old style NiCad or NiMH. Li-ion do not have a 'memory' and cannot be discharged all the way down or they will never charge again. It is best to leave a Li-ion battery on the charger when you can because they only have a finite number of charge/discharge cycles.

Here is a link if you don't want to take my word on the subject:
http://news.tigerdirect.com/2008/02/08/fact-or-fiction-leaving-your-laptop-plugged-in-all-the-time-will-harm-your-battery/

http://www.howtogeek.com/124715/should-i-leave-my-laptop-plugged-in-all-the-time/