The variations of challenges that the player will encounter in the I, Zombie mode are many, and the game provides different zombies are different levels. There are, however, a few basic tactics that I will cover. These tactics focus on endless mode - they might also be possible in the previous levels, however, if the same zombies are available.
One of the most common tactical decisions that must be made is if it is worthwhile to assault a lane directly or from behind. Assaulting from behind using a Miner Zombie can seem like a good idea, but remember that the Miner Zombie won't eat the brain in the lane you're attacking. This means that you'll also need an Imp (at the least) to eat the brain, which puts the cost at 175 energy for using this tactic. You'll still be ahead overall, but it may be less expensive to use two Cone Zombies.
Using a Bungee Zombie is also expensive. Sometimes you'll have to do it to get rid of a plant which is causing havoc across multiple lanes, like a Magnet Shroom that will remove picks from Miner Zombies before they can get to the rear of the row, but don't use the Bungee Zombie just to take out a single Snow Pea.
Bucket Zombies are extremely good at taking out lanes which have lots of Peashooters but not much else. Cone Zombies can do the same job if there is only one or two Peashooters. If there are Chompers or Potato Mines in the way of your Bucket and Cone Zombies, use well-timed Imps to tie up the offending plant. You can also use Pole-Vault Zombies to reach a plant behind a Wall-nut or other obstacle.
Stay away from the more expensive zombies whenever possible. There are few situations, for example, where a Football Zombie is going to be much more useful than a Bucket Zombie.