[CentOS] When should I reboot?

I reboot when I yum update to a new kernel or systemd, which seems to come out about once a month. Should I do it for this week's glibc? Is that "core" enough to justify a reboot or should I wait for the next kernel update? I know the glibc update was mainly to handle the new Japanese calendar, so that shouldn't affect my usage. So my question is more about how shared libraries work and whether anything bad would happen with different forks of running services (mainly the mail suite with dovecot and the various content scanners launched by sendmail) running different versions of the library based on when they were started.