Simple:- Every time, all the time. Anything that adds weight to the Aircraft will cause it to burn more fuel. My lunch or that extra fuel I added for mum and the kids.
Anything bigger than a 737, then consider it.
Anything smaller, don't bother.
From the CX manuals:-- The penalty for carrying additional fuel is between 2.5% and 4.5% per hour of flight, dependant on aircraft type.
Just as topbunk said above.
