well, u gotta know something bout the boot process.
after the kernel loads, it executes a few scripts.
the first one is /etc/rc.d/rc.sysinit which will set up system-wide basics like hostname, networking, system checks, mounting disks/modules, etc. Then it executes /etc/rc.d/rc.local which will set smaller stuff like banners n such. After that, depending on the runlevel ure in (/etc/inittab) it will execute starting/killing daemons from the corresponding /etc/rc.d/rc#.d/.
Stuff like setserial's u best add towards the bottom of /etc/rc.d/rc.sysinit, since its got to do with the system and needs only be run once at boot.