If there isn't a worry about cable fishing and space constraints, you can do either or. The whole reason why you would want to connect a single cable to a Plasma that was goign to be mounted over a mantle would be for simplicity purposes and to truly make your setup a surround sound setup. Some people say "well, sometimes I don't feel like watching a movie in surround sound and I only need my tv speakers", in cases like that, some people connect all their sources to the tv and then to the receiver from the tv. The only downside to it is that sometimes you can't send a dolby digital signal to the receiver because in most cases you're only sending a stereo signal from the tv through the rca left and right cables from the tv's audio out.
Another thing you can do is "double dip", but that can get expensive. By "double dip" I mean that you can connect everything to the receiver and then use an hdmi connection from the receiver to the tv...
And then, for whatever sources you want to use without the receiver sometimes (like the cable box), you can connect it to the tv and the receiver at the same time.
Let's take an hd cable box for example.... Say the box is connected to the receiver using hdmi, right? While the box is connected to the receiver and the receiver is connected to the tv through hdmi, you can run a set of component cables from the cable box (since the cable box will have 2 or 3 different type of outputs) to your tv and pluging them into a different input and still leaving the hdmi connection plugged in.
Still with me? Good..
So if you want to watch tv without turning the receiver on, you can do so and then all you have to do is change inputs on the tv and voila!. Like I said, it's the expensive way of doing it but that way you can have the best of both worlds.
As far as I know, there is no signal loss when sources are connected to the receiver and then to the tv. Receivers are meant to do this kind of task, so there is an emphasis picture quality. You might experience some signal loss if you're using an analog source, like a vhs deck but not necessarily depending on the quality of the equipment.
Line conditioners....Well, line conditioners are nice but not essential. Their job is to regulate power and provide you with clean power. In my opinion (and that's a personal choice), I would purchase a nice surge protector and leave it be. Some people claim that a Plasma needs a line conditioner and so do rear projection tv's because if a storm rolls around, your equipment is safe. Let's face it, how often does a storm hit? (depending on where you live, of course).
It's a simple rule, if you have nice equipment and you know a storm is coming, then unplug the surge protector from the wall and you'll be fine. The same principle applies to computers at home.
Remember when people used to have modems in their computers to connect to the internet by dialing in? If you used your pc in a storm, chances are your modem could get fried, and some did. It's just a matter of common sense.
I purchased a Monster Powerbar 2100 one ebay brand new for $176 (they run about $350 at the stores), and that's more than enough for me. It tells me what the voltage is at and whether my current outlet is good enough to handle everything plugged into it.
Keeping in mind all these things, if you're still interested in purchasing a line conditioner and want to spend the money on it, do it by all means. What matters most is that you feel comfortable knowing that your equipment is safe.
Please let me know if you have more questions, I'd be glad to answer any more questions you may have.