I agree, walking the bases loaded is a bad idea. Can't count the number of times I have seen the winning run get scored on a hit by pitch.
The Expected Runs Matrix gives you some answers. Basically it tells you (based on research), that intentionally walking anyone is a bad idea, period. However, if the winning run is on third, I don't have a problem walking a guy to put him on first. If there are none out and bases empty in that position, the likelyhood of the team scoring the run is very high.