Dave
Nickel silver, unless you have a way of work hardening it, is probably as hard when you get it as it will ever be. The rolling process of making it into sheets work hardens it. Nickel silver is a good choice for guards, it is fairly easy to work, doesn't corrode too fast and doesn't cost an arm and a leg. Heat treating of copper alloys is usually used to soften the material after it work hardens from forming. So finally an answer, no don't heat treat nickel silver guards and butt caps. If you use an iron alloy then heat treating can be used, usually to bring out color or design in pattern welded steel.
Yes nickel silver does scratch easily, it is also fairly easy to polish back to an acceptable finish.
Jim Arbuckle
PS Thanks for filling out the location information in your profile, its nice to know what part of this rock you live on.