Incidentally, at what temperature should you be concerned? I typically set my fan to go faster once it hits 50C, but I crapped myself the other day when I came back into my room and it was at 83C. O_O
What was it doing to get to >80°C? I've seen this occur in my 2008 Black MacBook (up to 92°C, max - processor temperature only, the other sensors were 20° lower or more), but only when doing a massively parallel full-CPU-using task - encoding a DVD rip. This sort of temperature isn't uncommon for the processor
only, when you've got the CPU running at full capacity. If it was doing it when idling, then, yeah, there may be something wrong there. I'd also check, if you can find an appropriate program (I use the iStat nano widget), the other temperatures when it's up that high. I find that the heatsinks, GPU, etc. temperature sensors usually report a significantly lower temperature than the CPU.
Also, is it ok to use fan altering software? As in, am I shortening the fans lifespan? I'm trying to weigh up whether its better to let it heat up or make the fan go faster and wear it out.
It shouldn't shorten the fan lifespan significantly, if at all, unless you're running it outside the specifications. The fans do naturally spin up to their maximum rate (which, for my MacBook, is 6200RPM) when the temperatures climb high enough. As long as you don't exceed that (and preferably keep it a little lower for normal use), the fans should be fine - though it may hurt your battery life a bit. Also of note is that most fan control programs (I use smcFanControl) only set a minimum fanspeed -
not a maximum. So if you set the fans for 4000RPM and the CPU goes up to a really high temperature, they may spin up beyond 4000RPM - they just won't go
below 4000RPM.
For my laptops, I've always been fine keeping them in sleep mode, in a case, in a bag. Laptops produce a fairly low amount of heat in sleep mode, so it should cool off slowly, even if it was running hot before you put it to sleep. The one thing I'd recommend is to just check that the computer
does properly go to sleep before putting it away - just wait the extra few seconds until the sleep LED starts to pulsate slowly. I've occasionally had problems (no damage, thankfully!) where the computer hasn't properly gone to sleep when it should have, and it was ran a bit hot in the enclosed space on the trip home.