60-70 Degrees Centigrade is okay but that partly depends on the current air temperature. So the answer depends on how hot you expect it to get where the computer is being used. The temperature is never going to be a fixed value, it will be a differential between the processor heatsink temperature and the temperature of the incoming cooling air.
So long as the vents in the case are clear and the airflow paths as dust free as possible, you shouldn't have any problems. HP will have tested the machines in fairly extreme enviornments to make sure they keep working.
Lower temperatures tend to increase the reliability of components such as electrolytic capacitors which contain a liquid that can dry out if they are subjected to too much heat.
Unless you are experiencing problems that are clearly due to overheating such as the computer unexpectedly shutting down, it is better in most cases to leave well alone. If it ain't broke, don't fix it!