237SJ yes, they have sufficient of a 'go around' or two and for a diversion but that does not mean they fill the tanks 100% full . No aircraft carries more fuel than they have to, it is just plain economics.
The only time an aircraft may fill the tanks 100% if it is not necessary is if they are in a location where fuel is so cheap it saves more on price than it costs to carry the extra fuel, some Middle East locations have fuel so cheap that they fill up as much as possible.
There is another side to this question, the biggest risk in a crash is fire, and the more fuel on board the higher the risk and the more intense the possible fire, another reason why they fly with the minimum fuel needed .
I lived and worked in Saudi Arabia for 3 years, that is one place where fuel is so cheap aircraft fill to capacity, I actually tested the fuel for Dhahran airport every day among other jobs in the lab. Jet fuel in Saudi was 14 pence a gallon at the time so you can see why they filled up. Petrol was cheap as well 18 pence a gallon.