Reducing query cache for mostly INSERT-heavy database?
query_cache_type = 0
query_cache_size = 0


Seriously, turning off the QC is probably best. especially based on what you said.  Keep in mind that every INSERT (or UPDATE) to a table causes all entries in the QC to be purged.  Furthermore, if the QC size is large, (say, over 50M), the purge time slows down the write.

The ENGINE's cache is important to both reads and writes.  Generally MyISAM's key_buffer_size should be 20% of available RAM.  OR InnoDB's innodb_buffer_pool_size should be 70% of available RAM.