We have never purged Sales Orders, and I have a few questions about it:
 
First, is it a recommended practice to purge Sales Orders?
 
2. What is the advantage or purpose of purging Sales Orders?
 
3. When they are purged, do we lose any reporting or searching ability, or any history or detail information?
 
4.  Is it recommended to perform a backup before purging?
 

There are a couple of things you will lose by purging. One is the ability to copy a previous order. The other, which is somewhat related, is the ability to look in one place for the entire contents of one order. Those things may or may not be important to you.
 
Depending on your system and the number of SOs you have, you could GREATLY speed things up by purging.
 
My procedure is to purge all SOs more than six months old, at the end of every month.

The program purges all closed SOs. It doesn't know if they've been paid or not.