Sim instructor time does not count towards your totals, it is however interesting to keep track of your FNPT instructional time for your own record and it does "look professional" from an instructor point of view.
it is entirely up to you but I think if you decide to log it, shows you do care about what you are doing and sends a good message to a future employer.
I have about 500 hours Sim instructor time and i regret now not to logged them
cheers!!!