This is one of those threads that can go on a long time...'cause it all boils down to personal preference...and of course that varies with the individual.
Having said that, I prefer the power-saving modes reserved for the mobile platforms and NOT using it with tower setups...turning off all the wake-on states and telling the computer thru Power Management settings on Control Panel to turn off when I say off.
I run across many maladjusted power settings that prevent normal shutdown...and many ppl asking why their hard drive won't recover from hibernate or why their monitor won't come back from sleep.
MTBF for most modern hard drives is high enough that IMHO there's little or no difference when leaving it on all the time.
Memory get cluttered? Don't know that I understand you, there...
THIS IS PROLLY OFF-TOPIC:
As usual, have to say to cdogg and his rote response to shutting down Win98:
I've had users that were so illiterate they NEVER shut down...even when the system got so trashed they were just sure it was ALL messed up, and I'd remind them "the poor man's fix is a reboot!"
Well, they would, and then they'd come back and say "Now it says there's an 'Invalid system disk', what do I do?"
Once you got them going and asked them when was the last time they rebooted...invariably it had been since the last power outage, some time removed.
There's one regular poster on another ng that I frequent whose sig is how long he's run Win98 since the last reboot...so I'm maybe just doing what a friend of mine says..."In conversation, as in love-making, it's not good sometimes unless there's some friction!" (hopefully no one will be insulted with this frivolous post)
I know for sure cdogg won't LOL