References

It's good practice if you need a reference to your POD types, yes. Otherwise, passing those by value is the common approach.

In general, pass by value for small #s of arguments. I believe the arguments will be put into registers if possible, which is faster.

So, by value:

data in RAM -> Register (1 RAM access)

By reference:

ptr to data in RAM -> Register (1 RAM access)
Inside func: data at the location of the ptr -> Register (1 more RAM access)

Passing things like an int by reference isn't more efficient, if that's the question. The only reason you should pass by reference is if you actually want the function to modify the object passed in.

Also, remember that passing by non-const reference eliminates the possibility of being able to pass a parameter that can be implicitly converted to that type - that'll only work with by-value and by const ref.