If your are using your system @ default setting, there should be no problem with heat . However, there is some exception .
Processor : Up to date processor runs already cool (@ stock setting) so better cooling is not required in most case .
Motherboard : Just like processor in 99% of case motherboard do not require additional cooling (@ stock setting) .
Memory : same as above .
HDD : depend on model . For example, raptor's (10k rpm disc ) heat up and require a good case cooling (to prevent damage) , but most other models run about fine (good air follow is preferred : it is better to be safe than sorry when we deal with HDD and data) .
Video card : Here we have a large story , There up to date some card witch run fine (stock setting over 90 c) . But just like HDD it is preferred to have a good air circulation inside the case .
Finally cleaning the computer regularly (once in month or 3 months) will help your cooling .