Tombac is a type of low-zinc brass. The official formula for the tombac alloy used in WWII 5 cents is 88% copper, 12% zinc.
As for your other question, what you collect is entirely up to you. There's no "law" that you must only collect slabbed coins, or not. As far as I can tell, the main drawback to having some coins in your collection slabbed/graded and some not is they won't all be housed and displayed together, unless you buy some of those "slab-like" coin holders to keep your ungraded coins in. If you don;t think that will bother you, then buy what you like, and forget about the eventual resale price.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis