There is no way that I know of to do that permamently, but there is the built-in editing command "no caps on" and "no caps off".
After saying "no caps on" you can just say things like "seattle is not the capital of the state of washington, olympia is." All characters will appear in lowercase.
That the lowercase mode is activated can be seen in the status window like this:
Check out the vocabulary editor for more tricks.