wouldn't call is completely wasteful. If you can tie the finish, then you have the required amount of tail. (and then some) I always tie the overhand but don't worry about it when it comes undone (the overhand knot), which it always does.
probably the more important thing to do is to tie the fig 8 cleanly (strands always parallel) and then tug on each of the 4 strands of rope coming out of the knot individually to really chinch the knot down.
single bowlines = scary knot IMO. any knot that really needs a backup is not my knot of choice.