It all depends on what cleaning system you are using.
If you want to use a detergent in-the-tank based system then a higher temperature and pressure is handy on dirtier jobs.
If you are using a prespray-agitation system then, as they are usually free rinsing, a lower pressure and cold/luke warm water is sufficient.
I use an Advantage with 135 pump, cold water and two high power vacs and get very good results from EOT jobs.
It is very much horses for courses and sorting out what suits your own personal method of working.
When I started a 35 psi single vac machine was about all there was.
If you are doing miles of pub and commercial carpet then a large truckmount will come into its own, otherwise unless you are having problems carry on as you are and don't worry too much!