Usually you put stuff like that in /etc/sysctl.conf. IIRC the key is vm.swappiness.

Looking at the man page, I would think you are correct but I don't see a example on that setting.  I'll have to google for it I guess. 

Putting it in rc.conf does work tho.  I also used to have to adjust my fan divisor settings and I did that in rc.conf as well. 
 

IIRC you're using "echo 10 > /proc/sys/vm/swappiness". If it were me if would use vm.swappiness=10 in sysctl since in this case the control option is available, then fall back to /etc/conf.d/local.start for "echo 10 >" type stuff if there was no sysctl option.