Like Nancy, I have the impression from P&V's Low Carb Performance that sodium (and perhaps magnesium) might have to be supplemented but that potassium will stay in balance if the other two are. Here are the bullet points from Chapter 9 Fluid and Mineral Balance, page 79.

• Low carbohydrate diets increase the loss of sodium and water from the kidneys.
  
  Failure to adequately replace sodium adversely affects potassium balance and has several negative effects (e.g. fatigue, fainting, headache, loss of lean mass).
  
  The easiest solution is to consume an extra 1-2 grams of sodium per day in the form of 2 bouillon cubes or home made broth.
  
  Most muscle cramps are due to magnesium depletion in cells.
  
  Adequate magnesium intake helps prevent cramps.
  
  A 20 day course of slow-release magnesium supplementation effectively treats most muscle cramps.
  
  Dietary potassium and magnesium (as well as other micronutrients) can be increased by appropriate preparation of meats and vegetables.
  

They never mention potassium supplementation but they go on to point out that meats are an excellent source of potassium, noting that 4 oz of meat has as much potassium as a medium banana.

What I took away from P&V is that the attention to potassium supplementation may be mis-directed. For the most part if we take care of sodium, and make sure we get some potassium and magnesium in our food, we will keep electrolytes in balance.