Backup Question for Point-in-Time Recovery

I'm making a base backup with 9.1rc by following 24.3.3 in manual:
http://www.postgresql.org/docs/9.1/static/continuous-archiving.html
1. SELECT pg_start_backup('label');
2. perform file system backup with tar
3. SELECT pg_stop_backup();

But when I was performing step 2, I got warning from tar command that
file was changed when tar was reading file on some files such as
'postgres/base/16405/5659097'.  Will I get a good backup in this case?