Leave cassette tape deck ON using ANY source

A number of years ago I was told to always make sure the cassette deck is on, irrespective of the program source. It had something to do with the way a preamp works in conjunction with the cassette deck. Can someone explain to me, in laymans terms, why this is true? Also, if it is true for the cassette deck, what about the tuner and turntable and CD player?
I've never heard this, although it may have something to do with the idea that the signal from the sourdce is sent to the tape deck and the resto f the preamp circuitry at the same time. It may be that the tape deck will draw a different amount of power from the input signal when on and off. I have noticed this recently with my Wadia directly driving a pair of full range amplifiers and a Behringer DEQ equalizer connected to a set of powered subwoofers. If I play the system with the equalizer turned off, the main speakers sound dull. When I turn the EQ on, the main speakers brighten up and get louder, even thoughthe eq is not in line with the main speakers. For this reason, I am looking for a pre-amp with buffered outputs, which are electrically isolated from each other. I suspect the same thing could happen with a tape deck, depending on the design of the preamp and the tape deck.

The other sources would not be a problem, because they are switched on one at at ime by the selector knob, so that even though they are cabled up, they are not electrically connected to the system.

Bottom line, play some music through a source with the tape deck off, then switch the tape deck on, see if the sound changes. Keep the volume low in case there is a loud turn-on thump when you switch the tape deck on.
The problem is that when unpowered the input circuits of the deck can load down line level signal in the preamp, often resulting in a loss of highs. Whether or not this is an issue depends on the architecture of the preamp.

If the preamp has buffered recording outputs then the deck is isolated from the rest of the preamp circuits and you don't need to power it up unless you're using it. Otherwise it's best to power the deck up or disconnect it during listening sessions.

This is only an issue for devices connected to the preamp recording outputs, and doesn't apply to input devices such as Cd players, tuners, turntables, etc.
I would say Ghostrider nailed it.Frank Van Alstine will include buffered tape inputs/outputs as an option for this reason.I have heard that cassette decks have continuous motors running when turned on,but not being used so its quite mechanically wearing to have the unit powered up at all times.Digital sources are different in this regard,so Im told,good luck,Bob
I used to own a Teac reel to reel (I can't remember the model #), but it sounded, I thought, great, at the time. I played everything through my tape deck, ( monitoring the sound through the deck) the tapes electronics, it seemed, made everything sound better than the actual source itself. That may be why someone told you to leave it on all the time. It won't do much unless you route your signal through it, and, unless it is a really good deck, I don't think it will make things sound better. I've long since replaced that deck with a Revox A77, but have not used that in quite a while. I would however recommend that you try it with a good quality deck like a Revox, Akai, Teac, Studer, or even Sony or Pioneer reel to reel types. Cassette decks, though, where not of the same quality, so I'd consider giving it a try with a top of the line model, like a good Nakamichi, or equavalent. Running the sound through the analogue circuitry seemed to smooth things out making all sources sound less fatigueing. I no longer use my reel to reel setup or, even any of my cassette decks, but now, I'm curious again. So, I think I'll head down to the basement tonight, clean up my Revox A77, and try it. There. Now I've given myself a project!