What exactly is an "ideal" idle temperature for a CPU? Does it even really matter outside of extreme circumstances?

I've recently been messing around with PS3 emulation on my PC, being sure to monitor my CPU's temperatures using MSI center so that I don't turn it into soup. However, I've noticed that even when my PC is completely idle, the lowest temperature the CPU will get to is around 130 degrees Fahrenheit, with it usually being closer to 140 degrees Fahrenheit.

From what I've read online (which, I know, take everything you read with a grain of salt), most sources claim that an "ideal" idle temperature is around 85-120 degrees Fahrenheit, so my average of 140 made me a bit concerned. My PC is only air cooled, so I figured that could be part of the reason for a higher temperature. Also, my recently departed cat loved hanging around my computer tower, but her fur was basically falling out before she passed, so I also wondered if it was just time for a good ol clean.

Before I did any of that though, I decided to see how the PC did under load. I don't play many real hardware intensive games, but broke out Robocop Rogue City to see if it would give CPU any trouble. However, even on the highest settings, the temperature maxed out at around 170 degrees Fahrenheit, with it averaging at 150-155 Fahrenheit.

So, I guess I'm just confused as to how my PC is somehow on the high end of what is considered a "just okay" idle temperature, yet seems to running at near ideal temperatures under load. Are the ideal idle temperatures just way off the mark? Does it even matter if my CPU is running at around 130-140 while idle?