Well, as a motorcyclist who only has two tyres to begin with, checking pressure is a pre-ride routine. Always. There are some rules that we follow religiously.
Firstly, always use what the tyre manufacturer recommends. This is more for riders than drivers, safety, insurance etc reasons.
Secondly and most importantly, ALWAYS check on cold/ambient tyres for the reasons Daz already stated, variations due to usage.
Having said all that, a couple of psi here or there for cars is not going to create any issues. Even as much as 10 psi could go unnoticed by most drivers I found.
People who race cars/karts will be way more tuned to this. I remember in my karting days, a couple of psi could make a second difference on a single lap, but that is getting to the extreme end of the scale now...