fj12ryder is correct in that without backup data/information opinions are of little value. Now, back to the OP's question:
" If most Drs. & other medical facilities accept Medicare assignment is a supplement policy necessary? "
There are many legitimate non-profit charities. It just makes sense to know ahead of time about the one(s) you wish to support. There are many 'watchdogs' that may be used to determine the cost effectiveness of a particular charity.
Here is just one: Charitywatch.org You do NOT have to 'login' or be a member to use this one. Just use the search engine in the top rh corner.
Be careful as even some watchdogs are commercial ventures.