Although there is no toggle in options there IS a command line option to hide them.
Press tilde ~ (or remap the key in the keyboard control settings if you are using a non-US keyboard layout, then press that)
Press enter
Type in: HelmetVisibility <value>
With <value> being true or false
This apparently only affects only currently selected party members.
WARNING: Activating the console may cause achievements to be disabled for that game! (You can use the console to cheat in lots of ways). Some commands do effect achievements and some don't! I would recommend trying a new save just in case