We are planning a trip from Paris to Switzerland (Luzern), Austria (Salzburg) and then Italy (Florence). Will we need to travel through Germany enroute from Luzern to Salzburg -- thereby needing a five country Select Pass? Or will a four country pass work?
Also -- we're new at this, please forgive possible "dumb" questions -- how do we know which trains we'll be taking, so that we'll know which ones need reservations?
Yes, the rail lines going to Salzburg do pass through Germany. The trains may or may not stop in Germany but you will go through Germany to get to Salzburg from the direction you're coming from (west).
Your 3-country Eurail Select pass has to be used in adjoining countries. I could be mistaken but I don't think you need to bump up to a 5-country to get from Luzern to Salzburg as there are runs that do not go through Germany. Also, as long as you don't have tickets that end in a German city or town, you won't need the 5-country pass (even if you transition through Germany).
For reservations, only the local regional trains do not require reservations. These are the ones that stop at every stop. The faster inter-city, Euro City, Eurostar, Cisalpino trains do. Plan on reservations.