Hi, Rossco,
I know that some people do take the metro or a bus or even a commuter train to start further away, but you will never hear that recommendation coming out of my mouth.
It's totally up to you, of course, but the 20 km from the Cathedral to the lovely Alpriate albergue are really pretty interesting. First, you walk through the old moorish district, the Alfama, then past the old port and the tile museum, next to the Expo park along the river, under the amazing Vasco de Gama bridge, and then along a channel (admittedly not the nicest scenery Lisbon has to offer). If you are going to spend the night at the albergue in Alpriate, it doesn't make sense to cut this day short since it's already a very manageable 20. But if you wanted to sleep in and then just get close, you could go to Sacavem or someplace like that.
It'd be good to hear what more recent pilgrims from Lisboa say about this first stage, because my feeling is that a lot of the complaints had to do with the way the arrows used to take you. But I could be wrong about that, so maybe someone will fill us in.
I've added some of my pictures from that first stage from my 2008 caminho from Lisboa. Bom caminho, Laurie