Yes, this is one of the legal documents that should be completed ASAP, if you don't already have one. It's important to meet with an Elder Care lawyer as early in the process as possible, and make sure that your "ducks are in a row" BEFORE they are needed. I happened to go to a Legal & Financial Planning Conference just a few days ago, so if you go to my page and look at my Updates, you'll see what was discussed and recommended.

You can also find good information in the book "The 36-Hour Day" which is recommended reading for Caregivers, by most Alzheimer support groups.

Edited to add this note:
I have a POA for my hubby, who has Alzheimer's, but he's still in an early stage so I haven't had to actually use the POA yet.
I also have a POA for my son, who has a Traumatic Brain Injury plus Vascular Dementia, and that has been invaluable. It lets me take care of all kinds of paperwork on his behalf; take care of financial matters, file his taxes, etc.

I'm my mother's power of attorney and health care proxy. It's a needed item I believe.