Additional Safety Factors are only applied if a Performance Chart does not take them in to account. Most allow for the variables of Temp & Press (Density Altitude) and whether there is a Headwind or Tailwind.
Adjustments for Runway slope is not always given so a Safety Factor would need to be applied in this case - as previously stated check the graphs in the POH for the aircraft you will be flying - and for the exam study the one given to see exactly what variable is, and what is not, included.
As an extra safety margin, for CAT an additional 25% is required to be added to the calculated TODR and 43% to the calculated LDR.
However, because these are mandatory, most operating manuals for aircraft which are mainly used for CAT will have Performance Charts that already apply these (they will state on them whether they do)
For GA (including the PPL written exam) you will be expected to apply the CAA recommended Safety Factors of an additional 33% to the calculated TODR and 43% to the calculated LDR.
The reasons for this are as stated by BPF above - along with taking in to account the 'possibility' that the aircraft Mass, the Temp, the Press & any Headwind are not necessarily exactly the values used in any calculations.
So, in your exam, if you calculate a value for TODR which seems to be within 1-2 metres of a possible answer; but there is also a possible answer that seems to be exactly 1/3 longer than the distance you calculated - Then the longer distance is the correct answer.
NB; Make sure you can easily convert between feet and metres as sometimes the Performance Charts will be in feet, but the declared distances/possible answers will be in metres.