Some times it depends on where you need to work as to where the best place is for the jack stands to go. Usually under each frame rail, across from each other. Sometimes between the front bumper and the front suspension or under the cab on the frame around where the front cab bushing mount is. Maybe both on the rear axle or rear part of the frame. Depending on what you are working on it's good not to have them in the way either! Some times I've put a 3rd or 4th stand up close to the frame but not fully supporting it in case the others get shakey for some reason, there will be a back up to catch it.
Depending on the job, I usually put them either under the frame rails or else under the lower control arm. If I remember correctly, there was a circular area on the control arm, that my larger set of stands worked well with.
Before I go under something supported with stands, I always give the vehicle a good shake test. I would much rather have it tip over while trying to than have it tip when I'm underneath it!