Adding a drop or two of Elmer's actually harkens back to the earlier days of modeling when the adhesives used by decal manufacturers were generally of much poorer quality, and particularly prone to breaking down (in the box, on retailers' shelves) with age and heat. Though I know there are modelers who still swear by it, it's pretty much unnecessary these days unless you're using really old decals.
(If you happen to run into decals where the adhesive has rubbed off or is otherwise ineffective, I've found that Future [or whatever it's called these days] is a more effective fix than white glue. Just my .)