This is a little different take on your question: I'm looking more at the "Should I shut down properly vs can I just cut the power?" question:
I can't find the reference now, but I remember reading an article back in 2003 about a large company where the IT department had done an internal study: they took two very similar departments and taught one of them to always carefully shut down the PC when the left of the day for security reasons. This was strictly enforced. The other department was instructed to leave them running under the ruse that IT wanted to be able to connect remotely for maintenance. Again, this was strictly enforced, and they would then kill the power to the machines each night. This went on for a year, and support costs for each department were tracked.
The result of the study was there was no measurable difference in support costs for the two departments between those that shut down the computers correctly, those that just powered them off without shutting down, or either department compared to the year prior.
But what does that really mean? I know there's at least one problem with the methodology — there should have been a third department tracked that was allowed to do as it pleased for a control, and perhaps another department where machines were kept running as much as possible.
We also need to be careful not to draw the conclusion that this means it won't ever cause a problem with your machine; it's very possible that powering off the computers without shutting down did break a few machines in the study, but this either did not measure statistically for the support costs (maybe it's more likely to happen near the end of a PC's replacement cycle, for example, which reduces perceived cost) or is offset over many PCs by something like wear incurred while shutting down properly. Also, PCs from 2001-2002 and prior (when the study would have been conducted) are different from those built today.
As for the question as asked: I usually leave my work desktop turned on. I like being able to pick up in the morning right where I left off the night before. I do something similar with my home desktop. My laptop, though, I try to remember to shut down much more often. Additionally, I like to make sure I shut down when leaving the office for the weekend on Fridays.
For the servers I manage where I work (about 13, hoping to consolidate to 6 via VMs in the next year, 3 or 4 the year after), I have a maintenance cycle scheduled every month where I can restart them if I want. Most months every server is re-started, but sometimes I'll decide not to interrupt them. My maintenance cycle comes over the weekend on first weekend following Microsoft's monthly Patch Tuesday.