Do I Still Need to Wear a Retainer After Invisalign?

Whether with wires or Invisalign, once the teeth are moved to the desired place, they won't want to stay there. After active orthodontics, wearing a retainer is the only way to keep them there. Over time, the bone will become more dense and retainer wear can be moved to night-time only.
It is as if you were given a gift of pajamas/nightgown that if worn every night would keep you skinny. Would you wear it? Even if it wasn't your favorite color?
The aligner materials are not strong enough to wear indefinitely. The should be discontinued after a month or two. The Vivera retainers are designed to last much longer.

Most doctors recommend that anyone who has
completed orthodontic treatment, whether braces or Invisalign treatment, use a
retainer to prevent teeth from gradually shifting back towards their initial
position. Because every case is different, please speak to your doctor to
confirm if you need post-treatment retainers.

Yes, you will need a retainer. Teeth naturally want to move back to where they used to be, which is why your orthodontist will recommend that you wear a retainer. Wearing your last Invisalign tray as a retainer is probably not a good idea. They aren’t made of the same material as long-term retainers.

You could theoretically wear the last aligner as your "retainer." However, after time this aligner will discolor, wear or loosen. 

The only guaranteed way to keep your teeth from shifting is to wear a retainer every night for the rest of your life.

A fixed or removable retainer are better options than your last aligner.

If you fail to wear your retainer, just be prepared to go through treatment again.

The most common Invisalign case in my office is an adult 35-50 years old that had braces as a child, never wore a retainer and now their teeth have shifted. 

The Answer is yes!

It is important to wear retainers at the completion of active tooth movement. Invisalign has a series of retainers that can be purchased called Vivera retainers.  They are very similar to the Invisalign trays although made of a somewhat stronger material.

When you purchase Vivera, you will receive 4 retainers per dental arch that will be sent to your home address at the appropriate times over a 2 year period.