It's generally a good idea to keep an eye on its temperature to make sure it doesn't get too high. Some apps can do this, such as Battery Bot and CurrentWidget (both open source) though they report the battery temperature. As a rule of thumb, I consider 38 °C and above to be high, so pause what you're doing and turn off the screen to let it cool off a little first.
Extended exposure to high temperatures can degrade a battery and shorten its usable lifespan (both per-charge and overall recharge cycles). In addition, high temperatures also make batteries less stable (possibly even leading to explosions in extreme cases).
Also, never leave a phone connected to a power source for extended periods if it's already fully charged - it's bad for the battery. Yes, there are cutoff circuits designed to mitigate that, but there's only so much they can do.
