Case Trim Frequency Neck Size/Form Fired

[orangeparkplink]

I am reloading 30-06 rounds and only neck sizing them after they have been form fired. Since they are form fired and will only shoot them out of the same rifle, how often should I trim the cases? Neck sizing is supposed to help your cases last longer, but I read that you should not trim cases more than 4 times (generally). If I trim them each time I reload, am I reducing the case life? If I do not trim them , how can I ensure that the rounds are uniform length?