As most, if not all, PC's today have the ability to shut down virtually all of the components in them, leaving one on won't use much more power than turning it off. In fact, one article I read showed that turning off a computer, but leaving it plugged in, could use more power than the sleep or standby modes. I think this had something to do with business PC's and Wake-On-LAN network cards, but it was still interesting.
Windows NT, 2000 and XP are much better at handling memory and errors than Win98 and below
were, so don't need reboots to clear the memory. Unix based systems like MacOSX and Linux are also really good this way too.
Unless I will be away for a few days, I just leave my computer on. During a bad thunderstorm I may turn it off too. It's just easier to sit down and start working than having to wait.
goates