power and wire questions

           Where does everyone usually  mount their distribution blocks,  under the hood or interior?          Also I was wondering where people usually tap 12V to power relay coils and other various lower current devices (l.e.d.'s, dome lights, etc.)?          If a distribution block has no smaller than 12 gauge ouputs how could I us a 14 gauge wire?               I I have a headunit that appears to have 14awg or 12awg leads and says to run power directly from battery or fuseblock and not factory radio power behind the dash.   There is a 20 amp fuse inline for the radio.  Ok fine thats no problem.   Crutchfeild says to  use a 10 gauge wire for power and ground.    Would using a larger gauge wire than the one on the headunits power harness be of any benefit since I'm not going to fuse it any more than 20 amps?                         Help is appreciated