Mishna B'rura (61:16) cites a difference of opinion regarding whether one should respond "amen" to the shatz's (leader's) blessing immediately before "Sh'ma" or should, on the contrary, finish saying the blessing along with the shatz in order to avoid saying "amen".
I know of people who act according to one view, and of people who act according to the other.
But I also know of people who finish saying the blessing along with the shatz during shacharis (the morning prayer service) and answer "amen" to the shatz during maariv (the evening service). Does anyone have a source or reason or provenance for this?