It all depends on the matchup. Against say, the Bulls, obviously Cousins will guard Boozer and Whiteside on Noah. So in effect, Cousins is at PF and Whiteside at C. Against the Celtics, it's probably best to have Cousins guard Perkins and Whiteside on KG; so the position would be switched. Then the coach can always switch assignments in the middle of the game, say have Cousins guard KG and Whiteside on Perkins. When the Kings encounter a team with a stretch 4 like Jeff Green, Diaw, Dirk, etc; they may have to sit Whiteside and bring in JT, Donte or Landry to do the job (yes, I do look at Donte as a combo forward).
When all said and done, positions are just names, it's how you use the five guys on the floor that's important. The Bulls in its heydays would put Jordan, Pippen, Ron Harper, Rodman and Kukoc on the floor. Try to wrap your head around who's the center/PF on that lineup. As long as the pieces compliment each other, who cares who plays what position.