I have been bothered by this warning a lot as well. While I do understand (and appreciate) that there are cases where this is a useful hint, which could potentially lead to bugs in your code, there are also rather benign reasons, for perfectly valid code where this specific warning is rather superfluous.
For example, I have tons of code where procedures/functions start something like this (just a very simple example)
Procedure ReadHeader (Var F : File);
Var Buffer : Array [1..512] of Byte;
BytesRead : LongInt;
begin
BlockRead (F, Buffer, 512, BytesRead);
This will result in the warning issued for both the Buffer and BytesRead variables, though these variables are assigned some value by the BlockRead procedure, but the compiler here doesn't check on the var return values and hence doesn't consider this a sufficient initialization of those variables (though it is as good as it gets).
So instead of using dummy assignments as some people suggest and thus unnecessarily bloat up the code, I will be using the {$PUSH} and {$POP} to locally disable the warning.
{$PUSH}
{$WARN 5057 OFF}
BlockRead (F, Buffer, 512, BytesRead);
{$POP}
Using the {$WARN 5057 OFF} at the beginning of the file, I do not consider a safe option, as it would indeed mask cases where a later used variable might accidentally not have been properly initialized/a value assigned...