Yes, this is a personal decision -- but here's some information to help you make yours. If you're starting from Lisbon, I'd estimate that 85% of the way to Porto is on pavement. From Porto on, it's less, but there's still a lot of pavement. From Lisbon to Porto, and from Tui to Santiago there aren't really any huge ascents or lots of uneven paths (I haven't walked from Porto to the border).
I know that the more pavement there is, and the less uneven the terrain, the more shoes seem to be indicated over boots. But I, like Michael, always wear boots (low boots, but more substantial than shoes would be).
Laurie