You could consider to never shut down your iMac ever again.
I decided to only log-out of the system and from what I've experienced it keeps my iMac running smoother, quieter and quicker than shutting it down and restarting it every day.
Last week I had shut down my iMac after an extended period without restarts. After the restart applications like Safari and Mail started up much slower than with a simple log-out.
Normally my iMac is very quiet, but after the shut-down and restart it made much more noise.
I had to repair the permissions and run some maintenance scripts to get the system back to the quietness and the speed it had.