From the University of Minnesota Veterinary Diagnostic Laboratory

Can my E/N dog have episodes of EIC?

Our testing to date has identified more than 1,000 dogs with the E/N genotype. Approximately 96% of these dogs have no signs of EIC or any type of collapse, while approximately 4% have been reported to show some signs of collapse or intolerance associated with exercise. The vast majority of these collapses can be attributed to other medical conditions, or their signs are not consistent with the classic signs of EIC that start with wobbliness in the rear legs.
Similarly, approximately 5% of all dogs with the N/N genotype are reported by owners to show some signs of an exercise-associated weakness or collapse. Again, this is likely due to other causes and is not classic EIC. Thus, we feel there is sufficient evidence to state that carriers of the EIC gene are no more likely to show signs of a collapse than are clear dogs, and that any collapse symptoms they do have are very unlikely to be EIC.

In other words, there are many possible reasons as to why a dog can to collapse during exercise, and the mutant EIC gene is present at a high frequency in the population. It appears at this time that there is no association between carrier status and EIC.
Should I only use N/N dogs for breeding to prevent the possibility of producing E/E pups?
Although using only N/N dogs would more rapidly remove the prospect of EIC from future generations, it is very likely not the best approach to dealing with this problem, and we do not recommend this approach. Many excellent dogs in all other respects are E/N or even E/E, and to breed entirely away from their lines would potentially lose many of their finest attributes that people have come to accept, demand, and cherish.

Our data to date indicate that only E/E dogs are documented to have exhibited the classic signs of EIC. There is no chance of producing an E/E puppy if it is known that at least one of the parents is N/N. A breeding program that utilizes E/N or even E/E dogs can be logically implemented by mating to N/N dogs and retaining E/N or N/N puppies for future breeding that also retain most or all of the other highly desired characteristics. In general, we recommend matings that produce fewer carrier (E/N) dogs in each successive generation.