Very short answer: it depends on your skill levels and what weapons you're using.
Torso affects melee skill and dodge change, so stacking a lot of backpacks or duffels will throw low-skill practitioners off-balance and negate your dodge based on how overloaded you are. The penalties listed per body part come directly out of skill levels, so if your torso encumbrance lists melee penalty of -3, you need 4 regular character ranks in melee skill to fight at effective melee rank of 1.
The different body parts with encumbrance is something like this:
Head: Only determines how many hats you can put on your grand pile of hats, and warmth.
Eyes: Perception penalty for ranged combat (and trap detection!)
Mouth: Slower movement speed
Torso: melee skill + dodge skill lowered, swimming takes longer
Arms: Ranged combat penalties
Hands: reload speed penalties, throwing penalties
Legs: movement speed penalties, dodge penalties, (swim penalties?)
Feet: more movement speed penalties.
Ranged-only characters don't care much about torso encumbrance, melee wants a lot of armor on the arms and legs to block shots with martial arts. Little o' both kinda need to run light on everything.
Encumbrance is calculated by (sum of every clothing encumbrance value) + ((number of items worn/2) - 1) I believe.
So if you have a tank top (fits), trenchcoat (fits), backpack, and MBR vest (kevlar) (fits), your encumbrance would be (0+ 0 + 1 + 1) + ((4/2) - 1) = 3 total encumbrance. I don't have that gear handy right now so... someone correct me if I'm wrong?
Generally lower is better until you're an unstoppable murder machine that can soak the skill penalties for fighting in heavier stuff. Practice, practice, practice!