For the avoidance of confusion - and nothing else!
As you open the throttle one of the first things that should be checked is whether the aircraft is making the 'full power' static rpm. With a fixed pitch prop and at the density altitudes we get in the UK that will be in the range of 2100 to 2400 rpm across virtually all spamcans. The specific rpm for your model/year etc should be known to you.
If you do not get that rpm then shut the throttle and find out why. Could simply be the carb heat left out - or the engine may be knackered. Over reading is rarer but if for instance an O-200 has more advanced timing the rpm goes up - but only some O-200s can withstand this without cracking!
In the approach configuration the rpm is whatever you need. If could be anything between the idle and near full throttle depending what else is happening - so as long as the engine is running, the tacho will tell you nothing about the state of the engine.
Have a look at a couple of recent accident reports where deterioration of engine performance has had a major influence - the recent Sandown Cherokee one springs to mind.
It is unfortunately a fact of life that the standard of and content of, much of what you are taught during a PPL does not give you the information you really need to operate outside a club environment.
If the club are also 'sailing close the the wind' think seriously about going somewhere else.