Is shutting down an iMac or Mac mini really necessary? In today's age with low power standby modes &amp; the advanced OS X that is Mountain Lion, it is simpler than ever to go for long periods (perhaps months) without shutting down your iMac (and possibly Mac mini too).

Is it really necessary to perform a complete shut down of your Mac (and what considerations if any need to be taken account of)?
The benefits of shutting down vs. sleeping is an oft-asked question. Over time, reasons to prefer shutting down have either been resolved or significantly ameliorated.

For instance:


It used to be that shutting down early macOS (then OS X) machines caused issues with timed log clean-up routines. This has been resolved with the OS recognizing when routines haven't run and rescheduling them.
Similarly, long-running systems used to exhibit instability relating to memory issues. Shutting down the computer cleared the memory, temporarily resetting things. However, memory management has subsequently improved in later versions of macOS and this is no longer a significant issue. 
A shut down system used to be significantly cheaper. However, advances in hardware and software have improved the ability of Macs to sleep efficiently. In 2020, a 27" iMac with Retina 5K draws 0.27W when shutdown and 1.25W when asleep (compared to 40W when idle) (source). Apple prepares environmental reports on all its devices extending back several years that you can check. Copies are available from the Environment section of Apple's website.


In addition to these changes, features like Power Nap improve the utility of sleeping Macs.

For me, I set automated sleep and wake times and almost never reboot.