Short answer: With a dead PRAM battery, most soft-power Macs will retain PRAM settings if left plugged in (and the main power switch in the ON position) because they are provided trickle power. All hard power Macs with a dead PRAM battery will lose their PRAM settings if switched off.
Long answer: Except for weird old boxes like the early II series, a soft-power Mac (including PowerBooks with a good main battery) will always receive power as long as they are plugged into the wall (or have a good, charged main battery) and the main power switch (usually only on compacts) is ON. This is because a low, constant voltage is required to enable the Mac to turn on when the soft power button is pressed, so the engineers figured 'why not give the PRAM battery a break and just draw from this voltage when available?' This has the side effect of extending the life of the PRAM battery because the battery does not see a draw as long as the Mac is plugged in.
The early II series used an odd arrangement that required two batteries, one for PRAM and one to run the soft power circuit, so they were oddballs and won't boot at all if one or both of those batteries are dead.
Of course the hard power Macs do not have a steady trickle from the power supply so they will almost instantly lose their PRAM settings if power is removed. I'm not sure why modern Cuda-equipped Macs get weird about booting with a dead PRAM battery. I assume it has something to do with the Cuda's internal processor booting cold at power-up and failing to initialize properly, thus keeping the rest of the machine from booting without a hard power double-tap or punch of the reset button.
One thing to consider if you're leaving your soft power Mac plugged in all the time: they constantly draw power (obviously) and therefore are subject to some stresses even if they're not being actively used. For example, shut down your soft power Mac, come back in an hour, and feel the case near the power supply. You'll notice that it's still warm, and this is because it's still operating to supply that trickle power. This not only wears components out faster but also exposes the power supply to incoming voltage transients that could damage it. Thus I recommend not leaving your soft power Mac plugged in 24/7 just for the sake of preserving PRAM unless you're also using it on a daily basis. If it's a once-a-month hobby thing, just reset the date and time the next time you use it. These things aren't getting any younger.
