Depends on the population statistics used and the relevance of the statistics to the particular population of passengers (or baggage) on any given day. This applies to any use of presumed weights including, for example, fuel loads.
(a) generally, for a given country, say, there will be national statistics which can be used to determine a rational set of standard weights.
For instance,
(i) the original 170lb weight generally used for adult males was derived from North American military population studies back in the 40's if my recollection is correct.
(ii) the current Australian schedule,
CAAP235-1(1), dates to a study by a CASA (or whatever the name was that week) engineer by the name of John Klingberg. The study, based on national medical data, was quite detailed and gave a large sample base for the sums.
(b) providing that the population of passengers in question on any given day is consistent with the population in the sums, the use of standard weights should be reasonable - with two principal caveats -
(i) compatibility - if the study population is, say, national, then don't expect an aircraft load of sumo wrestlers or jockeys to present sensible data
(ii) due to the shape of population distributions (ie considering standard deviations) small passenger loads would be expected to show far greater variability than larger passenger loads.
If I recall back to the 70s, when I was flying F27s, we often elected to weigh the passenger and baggage loads in lieu of using standard weights. I cannot recall a single case where the total weights were inconsistent with standard weights.