You're still wrong at least when it comes to USPSA.
The course of fire begins at the Make Ready command.
There are certain things that you cannot do during the CoF without a penalty. One of those actions that will incur a penalty is lowering the hammer of a selective action DA/SA pistol to any position except all the way down when competing in P or CO. The penalty is a bump to open.
The start signal, in this particular case, is meaningless.