No, it is not safe. I am using Drupal 7.
I use quiz module on my website, and I noticed there at least were two error when I emptied all the field_revision_*
one with variable error which I do not really remember, and the other one was the questions were disappeared.
Oddly enough, answer options were there, but all the questions were gone. I did not look into more details, I put the field_revision_* tables back and it started working properly.
All I can say is if you use quiz module and go to myPHPAdmin and truncate these tables, you will get errors. If you don't use quiz module, I don't know. but it is not bulletproof. I would recommend using some proven modules to clear your revisions.