A pure 64-bit environment is just fine for almost everything I needed. I resisted a multi-lib setup with Slackware 13, but decided to go multi-lib with Slackware 13.1. Everything has gone very
well. The biggest thing I gain is Wine, running 32-bit Windows programs.
One tip, grab Eric's packages for multi-lib
(if you want that) to make things a lot easier.
After I installed the mutli-lib files according to Eric's the instructions, I had to make compat32 versions of a couple of updates that happened after Slackware 13.1 was released: seamonkey-solibs, libtiff, and libpng.
I use the Nvidia driver Slackbuilds.org package, so I also made a compat32 version
of that package.