Not talking with evidence here, but to me it depends on how much grimdark you want. If you want max grimdark, then all human souls are taken by the chaos gods upon their death and tortured eternally, making chaos stronger and its eventual victory inevitable. Yay.
If, like me, you want a little more hope than that (especially for the guardsmen, like me), then you may believe that most human souls are beyond the notice of chaos and just simmer away, or that they actually get to go to the emperors table and have some fun times with all the old heroes (in must be a bloody big table).
Obviously this involves Imperial humans, chaos, tau or other aligned groups may have who knows what happens to them. Chaos being consumed by their deity of choice seems common though.
If we assume that the table exists, then it mainly seems to depend upon the person, their situation, and the state of the battlefield after they died. In wrath of Iron
Another spoiler for Imperial Glory:
To my knowledge that, and miscellaneous texts about being lost in the warp are all we know about souls.
So if you die in the warp, or in conflict on a daemon infested area, your odds of being sent to the warp would likely be higher than if you died after a greenskin knocked your head off. If the daemons and chaos forces beat you and commence fun torture times, I would say eternal torment is basically guaranteed.