Lightroom does not simply access the raw files. It also uses the LR Catalog to store previews and adjustments. This is why images have to be imported into Lightroom. Of course an image can simply be added to the catalog during the import process without being copied or moved. LR is not a file browser, so you can't simply browse images in folders.
Yes, I suggest you do use a single installation on your main machine first so that you can become familiar with the way lightroom works. In the meantime, if you are out and about and need to import images into your laptop installation, go ahead. You won't lose anything.
It is worth gettng your head around how LR works before going too far, but you'll find there is very little you can do that cannot be rectified subsequently. When you come to edit (develop) images, remember that LR only stores the adjustments and applies them to the raw image. It doesn't change the image at all (you can't directly edit a raw image anyway). When you want to use the edited image elsewhere, you simply export as jpg or tiff. You can reverse any adjustments any time.
If you are new to LR (or even if not), I would suggest you may consider buying Martin Evening's excellent "Photoshop Lightroom 3" book. I'm looking at mine right now and it sprouts little post-it notes which I've added when reading it away from a PC and come across little gems of information.
I hope this helps - As you can probably detect, I'm a Lightroom convert!
Steve
