[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.