Mmm...if you can afford it, though you have also should consider the negative contribution to the environment of "wasting" electricity.
Don't get me wrong, I'm not trying to preach some extreme "green" mentality here....I mean, we're modern folks, and living with the comforts that electricity provides, is important. I hate the people who don't have AC and instead sweat their asses off all summer or who have it but will "economize" by only turning it on when it's like blistering hot (I hate sweating indoors).
I'm just saying that when you have electricity being used up by something that is not actually being used by you at that time...is kinda wasteful. You can instead use those kW hours to run that AC a little more.
In my studio...I couldn't leave shit on 24/7...'cuz it ain't just a computer and a couple of LCD screens that I'm using....though even a PC tower and a pair of 24" LCD screen when running, will burn up a lot of electricity. I'll leave it on all day, even if I break for a couple of hours, only because it's better for the gear than adding more on/off cycles to it....but when the session is done for the day or night...everything gets shut down. Not to mention, it certainly WOULD add to my electric bill.
I may not be back in the studio for a day or two...so why exactly is it on...and frankly, I don't think it's such a big deal to power things up and set up a couple of mics. I mean...there's rarely a situation where I'm running into the studio with some "hot off the presses" idea for a song, I just gotta hit REC and roll as soon as I'm in there.
I don't know who really has that kind of urgency...and I actually like the process of getting set up, because the whole time I'm doing that prep, I'm also thinking about what I am going to record, how I want to approach it, etc...etc.
The other reason I don't leave stuff needlessly powered up 24/7...safety from power spikes, brown outages, etc...from storms or whatever....and I simply don't like leaving gear powdered up and unattended. Again...I'm dealing with a bunch of rack gear, tape decks, console...etc...it's not just a computer or two that is powered up....and I have a few layers of protection in place, but I still
Like in the power-up sequence thread here in another forum - studio gear habits is a deliberate process for me.