It depends really!
If your in the middle of watching something online like streaming a movie then you can let it sleep (I do it sometimes) otherwise it saves your battery life and electricity if you turn it off.
If it takes lots and lots of time to start up and shut down there are legitimate ways to make it run better which I can help out with if you like, if not though then turn it off
First off, the Hardest thing on any electronic device is the Power on cycle. This is due to the High inruch current which can be 2 to 4 times higher than the normal current.
As to which is the best practice there are ones that recomend each.
My practice is power on when first needed and then only power it off when 1) Leaving the house for more than two hours or two when I retire for the night. My Systems use a SSD and Hibernation is DISABLED.
My wife leaves hers on 24/7 unless she is gone for more than a couple of days. Have NOT seen a difference in problems between hers and my computers. Her system uses a HDD and she can not stand the LONG time required during a boot.
Have a laptop at work it is left on 24/7 and has been for the past 2 1/2 years - NO, I would not do this for my laptop.
My recommendation is Power on when first used and power off at bedtime.
I do different things for different PCs. My main gaming PC is only turned on when I'm using it, or when I have it processing something (transcoding, etc). Even though I made an effort to use somewhat efficient parts and settings, a high-end PC with a decent GPU, OC'd proc, big PSU, etc is going to eat power. So it's probably on an average of 6-8 hours per week (not much time for gaming these days) all in one rush then gets powered off.
My laptop on the other hand is my main email, internet, chat pc - it is powered on 24/7 unless I'm leaving the house for more than one overnight. And I have hibernate/sleep disabled on it, so it's on on. It even only gets restarted when I absolutely have to (usually because of MS updates).
My media center I usually set to "sleep" - that way it comes back fast (even though it would anyhow with an SSD) and isn't using much power generally.