I tried ignoring mine when they did it and it didn't work. Infact my 13mth male will still do it when he's excited and he gets a very stern "No or No Biting", he knows exactly what it means and is just trying it on and over excited as well which we try to calm down at the same time. Only when he's calmed down and stopped the mouthing will I speak to him nicely or give him praise or say hello.
My 3 yr old bitch did it for a while but then suddenly stopped after us continually telling her "No or No Biting", think she just got fed up of getting no praise for mouthing.
I agree some behaviours are best ignored but mouthing/biting/nipping I will never ignore as I find a stern No does the trick - eventually