From the man page:

    NQDIR   Directory where lock files/job output resides.
            Each NQDIR can be considered a separate queue.  

Whether or not the default should be $XDG_CACHE_HOME/nq/<something> is a different question. For my own use cases with nq I like the current directory being used, but it would obviously be just as easy for me to set `NQDIR=.`.