PITR and tar

The docs recommend using tar to perform a base backup for PITR.

Usually, tar reports notices like:
"tar: Truncated write; file may have grown while being archived."

First of all, is the tar archive still safe if those errors occur?

Second, it seems that it can cause a bad backup to occur if you pass the
"z" option to tar. Instead, piping the output of tar through the
compression program seems to avoid that problem (i.e. "tar cf - ... |
gzip > ..."). I am using FreeBSD's tar, other implementations may be
different.

Are my observations correct, and if so, should they be documented as a
potential "gotcha" when making base backups?

Regards,
Jeff Davis