This is a common question- sliding in combat.
There is no mention of sliding in the rulebook. It states about charging:
"The unit must move
straight ahead, expect that, during this move, it may make a single wheel of up to 90 degrees. You are free to make this wheel in order to place your unit wherever you like against the facing of the enemy unit that is being charged, but remember that you must bring as many models into the fight as possible, from both sides! Therefore this wheel cannot be used to reduce the number of models in base contact,
unless of course you have no choice, and you need to wheel for the unit to avoid intervening units and impassable terrain in order to complete the charge." pg. 20
Later, when discussing combat reform, it says you must reform "keeping the center point of the unit the same". pg. 14
Because of this, since you cannot slide during the charge, or slide during a combat reform, and I didn't read any other time in which you can slide- Rules as Written; you cannot slide units to maximize base contact. I am familiar with FAQ's; and can't think of a time when sliding was discussed. HOWEVER I am less familiar with FAQ's than I am the rules. So it might have been FAQ'd and someone smarter than me can correct me.
Although unrealistic, I believe this is a reminder that warhammer is a strategy GAME, not a strategy SIMULATOR. Use this rule to your advantage. Do so by forcing some frenzied deathstar into an overrun pursuit. Have them overrun into your slightly clipped tarpit. Now the thing that is supposed to kill can only attack with one or two models, and your thing that's built to tank damage will sit there all game without any problems. If their are enough new lizardmen generals who would like a little tactic article on some basic tactics like this I would be more than happy to spend a day writing about a game and army I love

.