Unless your rod manufacturer screwed up (that happens), I generally oppose overlining with the exception of close-in work --30 to 40-feet or less.
The reason is the actual weight of the line compared to line weight aerialized during the cast. I've explained my reasoning before, most recently in a product update on Cortland's new 444 Precision Taper half-step llines. You might want to read the article:
The only line I've found that is an exception to the rule is Cortland's Classice 444 Sylk I also recently review. The 5-weight Sylk cast equally well on rods from 4 to 6-weight. Of course, the Sylk is quite different in its characteristics from other lines. That review is available at both Active Angler or Land Big Fish.com.
In the heavier weights - 9 through 12 - I always underline ... sometimes one but more frequently, two line weights. In fact, for long casting, my favorite rig is a light 12-weight that sends Cortland's old 444SL XRL a very long way.
You can find a further discussion of line weigjhts in a series (actually a chapter) from my book, Fly Fishing for the Rest of Us
, called, All About Lines
. It is available at either site.
Overlining is a gimmick used by a number instructors and schools simply because it shortens the time it takes to get the student to "feel" the rod load. I prefer to teach the student to watch the backcast and "see" the rod load. Down the road, overling leads to collapsed casts and broken rods...
Hope this helps.