If a pilot chooses to apply their own factor of conservatism that's up to them, it's probably exercising good judgement. And, it is true that aircraft performance can vary, as a result of tire pressure, inaccurate instruments (I'd be more worried about the ASI, rather than tach), more antennas, poorly fitted (or missing) fairings, or poor paint condition. A pilot is expected to fly an aircraft which has been maintained so as to be airworthy, and conforming to it's type design (about the way the manufacturer tested it). If the pilot willingly flies an aircraft with defects, they choose to take responsibility for any effect upon performance.
If a pilot cannot consistently apply skill to takeoff and climb which meets the mark of "average" when needed, some introspection is appropriate. It may not be the fault of the plane or its performance data, if the pilot cannot achieve "book" performance".
Otherwise, For my experience with Cessnas, and their performance data as published as far back as 1958, I have found it to be very accurate, and I have formally done the climb testing on many Cessna types. That said, I have done the testing on other types too, and for one, have found "optimistic" data in the POH (which was a separate document from the FAA approved Flight Manual, which did not contain climb data) - but this was not a Cessna.
Understand what you are reading. If you are reading a document which meets the requirements of "Flight Manual" and it is FAA (or other authority) approved, the performance data will be accurate, as it was gathered and presented in accordance with the certification requirements, and the authority was overseeing that. If you are reading AOPA material, sales information, a third party training manual or other such document, you may have erroneous data.
All that said, if you're used to a 152, and you start to fly an airworthy 172 with decent 152 skills, you will not be disappointed by 172 performance.