I do it all the time! I tend to like my flavor really strong, and sometimes my juices need a little "boost". I have never had it backfire, but I try to add flavors that are already in the juice.
ETA: I have been vaping for over 4 years. I tended to do it more when I was new, and checking out different vendors. If they didn't have the punch I wanted I added a little more flavor.
Now that I have found vendors that have the punch I like, I don't do it nearly as often as I used to
