I don't double my seams, but I will re-enforce the ends of the seams piecing blocks.
Just because with all the handling, setting, making rows, sometimes the seam start to unravel a bit.
I have quilts that have been lovingly used daily for several years and the seams are holding, the cheap flannel is getting thin though, and the dog is chewing on it... :( but it has been loved...
Kirsten