Saddle position for a long steep climb Various sources indicate that when climbing to slide back on the saddle. In about a month I plan to attempt Whiteface Mountain with its 13 km at an 8% grade. If I manage this climb I will be at my limit. Should I move my saddle back for the climb?
Most of the new schools of thought  suggest moving back or forth on your saddle is more of an indication that the saddle offset isn't set up correctly, than it is advantageous.  I would tend to agree.  If you find yourself constantly shifting forward or backwards, this likely indicates that the saddle isn't in the correct position.  

Old school fitting have been obsessed with KOPS (Knee over pedal spindle), but this was originally derived as a general method to determine whether or not a frame size was appropriate (i.e., if you couldn't get into KOPS, you needed a different size). (There isn't even consensus on how you even measure KOPS as everyone landmarks the knee differnently - this can result in over 1cm difference in offset alone).  Depending on your body proportions and bike position if you setup your bike to comply with KOPS this may have  put you into a sub-optimal position, which is why you may find yourself moving forward or backwards. 

Best advice is to go out with a set of allen keys and level and ride a reasonably long climb.  If you find your self wanting to move forward, try moving the saddle forward  (0.5cm at a time) then retry. Eventually you should be able to fine tune your fit.  The level is used to ensure you can keep the attitude of your saddle (i.e, nose up, level, or down) the same between trials.  Optimal saddle attitude is a topic onto itself (I feel a lot of "experts" have little understanding on the functional bio-mechanics of  hips and how this inter-plays with saddle attitude.)


  In about a month I plan to attempt Whiteface Mountain with its 13 km at an 8% grade. If I manage this climb I will be at my limit. Should I move my saddle back for the climb?


Honestly, I think gearing will be the most important factor for surviving 13 km at 8%.  I have done a 40 km ride at a similar gradient and being able to spin is a life saver (when I saw the "only" 19 km remaining I shed a tear). There are lots of long cage road derailleurs and wide range cassettes available now.  I would consider this as the most important change over fine tuning such as saddle offset.