Should boost be installed in /usr/ or /usr/local/ for bitcoin-core? When installing bitcoin-core, libbitcoin or libbitcoin-explorer the build requires the C++ boost library. Is there any reason why I should prefer to install boost in the /usr/ hierarchy rather than /usr/local. Or should I not attempt to link all my C++ projects to the same boost build anyway, and link to a seperate boost build (of a possibly different versions installed in different locations) for each project?
Libraries should be installed in /usr/include.

For Boost, the path to it should be /usr/include/boost. That's where the linker would be looking for the libraries.

Otherwise, you may elect to install the libraries in some different directory and symlink it to /usr/include anyways.