I don't think this is the case with centos/redhat. From http://www.centos.org/docs/5/html/Deployment_Guide-en-US/ch-autotasks.html:

"The cron daemon checks the /etc/crontab file, the /etc/cron.d/ directory, and the /var/spool/cron/ directory every minute for any changes. If any changes are found, they are loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is changed."