I still wouldn't worry too much about it. The majority of systems, pure 64 or multilib, have a /lib that contains 64-bit libs. On multilib ones it's simply a lib->lib64 link.
That means your binaries, 32bit as well as 64bit, should always link to /lib, not /lib64. That should be the best approach.
If I'm not mistaken, that's best practices. You won't, however, be able to compensate for, and magically make to just work, something where you've got the 32-bit compat stuff in the mix and not the 64-bit stuff installed (it IS possible... ). You'll get...odd...things to happen and there's nothing you can do to prevent them.