That's actually something I do before the first "make install". I first create /opt/trinity and lib, then I symlink a lib64 before I build tqt-interface. Why? Because I had a sneaking suspicion that something like that might happen with one or more of those packages, so I've been doing it all along. It shouldn't if you're installing binary packages though, that would be pretty sloppy of a maintainer to be inconsistent, unless they weren't specifically for your distro, or mixed.

I actually don't like "lib64" on a non-multilib system. There's just no need for that shit, but it's become a convention and a mixed bag at that. I don't want to have to remember to specify libdirsuffix for every build.

Arch Linux (and derivatives) makes lib64 the symlink to lib, and 32 bit stuff goes in lib32.