Each chord shape has it's own voice - tonality and that voice will stay pretty much consistent up and down the neck. The D shape, the C shape, etc.
If that voice doesn't fit what your ears wanna hear, change the fingering to another shape.
If you're playing a 2 or 3 note "chord", high up on the neck, and it sounds to thin for what you're after, use your un-used finger(s) and search around for the note(s) that will flesh out the chord and provide a fuller sound.
Regarding which strings to strum; start with the strings that have a finger on them and mute or don't play the ones that don't have a finger on them. Some positions will allow you to let open strings ring out, some won't, depends on the sound you're going for and / or the voicing the song needs to sound "right".
Not to be a smart ass... Use your ears.