tentative vs with grain of salt

tentative

noun
  • A trial; an experiment; an attempt. 

adj
  • restrained; polite; feeling one's way, not using full force. 

  • Uncertain; subject to future change. 

  • Of or pertaining to a trial or trials; essaying; experimental. 

with grain of salt

How often have the words tentative and with grain of salt occurred in a corpus of books? (source: Google Ngram Viewer )