First of all "critical" is a word with a number of definitions and can be emotive. When it comes to aircraft performance, there are aspects that can indeed be critical (extremely important), and certainly most aspects of mass/balance calculations have a high to very high degree of importance. When approaching a defined limit there is clearly a degree of criticality applicable. Nevetheless there are safety margins designed into most manufacturers and regulators performance calculations, and these margins in part allow for the "error factor" that may be inherent in day to day operations.
As has already been discussed in this thread, for most large passenger aircraft operations, the type of operation allows for the use of "notional" weights. These weights may well be individualy erroneous, but averaged out over a relatively large number of units should result in acceptable error margins. Ideally we would strive for the most accurate figure possible, but practically that is often not realistic.
To try and answer your question (I don't have figures for an A320/1) as it might apply to a Boeing 757 with 220 seats. If you assume a full flight with a 60/40 mix of males/females and 10% of that total are children between ages 2 and 12. There are a total of 230 bags. You would using notional weights arrive at a total payload of 19,744kg. If you used average adult weights your payload would be 20,392kg On a typical 4 hour flight with a fuel load of around 20,000kg this would give a take of weight of around 100,400kg. As a matter of interest this is still only 88% of the maximum permitted take off weight, or the maximum permitted payload.
If you had subsequently discovered an error of 10% in the payload this would mean that each of your adult passengers had actually weighed in at 93kg and all of them had bags averaging 14.5kg, you would actually be carrying a payload of 22,430kg and a take off weight of 102,440kg. The difference to your take off and reference speeds would be about 2kts.(2.3 MPH.)
On the question of crew and their baggage. Crew are taken into account in the basic weight of the aircraft (APS= Aircraft Prepared for Service mass.) Again standard weights are used. Crew baggage (except carry on) is an additional figure to be added and here the option to use either notional weights or actual weights is applicable, however it is added on and taken into account.
I hope this helps understand the topic. As you will appreciate I have tried to simplify what is a very complex subject. Some of the figures I have used are in substitution of "how long is a piece of a string?" But they should provide some guidance on how an important calculation can be tailored to allow for guesstimate figures being employed.