Both landing on the centerline and early on the runway are done for safety reasons. It gives the plane more margin for manouvering if needed.
You should land as far back as possible, so there's the maximum stopping distance, in case of any emergency.
[img=1000x750]http://static.panoramio.com/photos/original/1392039.jpg[/img]
This picture will really help you understand.
From bottom to the top we have 3 important parts of the runway.
First, the bit where you see the arrows pointing "up". You should NOT use this bit of the runway for anything other than taxiing. The arrows there mean that you shouldnt aim for that part of the runway while landing.
Then you have that zebra crossing looking like band across the runway. This shows from which point on the runway is safe to land on. You should aim to overshoot this by just a bit.
And then you see all the rubber in the runway which is where pilots landed their planes, as you can see, as far back as possible, but not as far as stepping on the zebra or the arrows...
About vertical speed at touchdown, 250 ft/min is very good.
As long as your plane touches down and doesnt bounce back into the air, and you keep it below 600 ft/m, its a good enough landing. Anything near 1000 ft/m and above is pretty much a crash

I hope this clarifies something.
Good landings ^^