I see many #CommonLisp library authors recommending and treating ~/quicklisp/local-projects/ as the only standard location that will be found by your system without further setup.
I use ~/common-lisp/ for that. I believe that it is more natural to use, and also more general, as it works on the ASDF layer, which e. g. Quicklisp builds upon. However, I just looked at the manual and discovered that this is a rather recent addition (3.1.2, 2014, 11 years ago).