Conventions on struct copying?

I'm still working on the struct object_id patches, and I had a style question. In several places throughout the code, we do something like this:

  unsigned char a[20], b[20];

  /* do some stuff with b */
  hashcpy(a, b);

I could implement an oidcpy that does the same thing.

  struct object_id a, b;

  /* do some stuff with b */
  oidcpy(&a, &b);

Or I could just write that as:

  a = b;

and let the compiler do the heavy lifting. Is there any reason that we'd want the function for that purpose, or is it okay to just use the assignment? I don't know of any place we explicitly copy structs like this, but I don't know of any prohibition against it, either.