A rod will cast the rated line, usually in a weight forward taper, best. At least in the opinion of the rod designer(s). Of course everyone is different so this is a subjective evaluation. Even then, for any given anglers, different lines may work better for that angler with the same rod under different fishing parameters (distance, wind, fly size, etc.) Unfortunately there is no objective way to determine which line will work best on a rod. This is why you will often read many opinions on what line best matches a rod or see someone state something like "Bumble Bee Rod Company's 9ft 6wt Stinger model is really a 7wt." You just have to try different lines. Over time, as an angler's skills develop, their line preference for a rod may even change!
With multiple rods in line weights 2 through 13, and fishing under many different types of waters and conditions over the years, I now have more than 75 fly lines in my collection. So I am able to try many different lines on rods. Since most anglers do not have this many lines, I also loan lines to other local anglers to try out and I sometimes swap lines with other anglers for a few hours or a day. Other local anglers, clubs, or fly shops may be a source of lines to try out.
For the most part, I find that most rods seem to work well over a wide range of conditions with a WF line matching the rod's rating, though I may prefer a different line, and I may go up a size or two or down a size if doing so will make using that rod more enjoyable for what I am doing. But I always test rods to start with the rated line.
In the case of the 6wt rod and bass bugs, that rod probably was not designed with bass bugging in mind. Think trout and panfish and smaller flies that a 6wt line carries well. So using an 8wt line with the greater mass will help carry those bass bugs and as long as you are not trying to carry 50 or 60 feet of line in the air on a false cast the rod will be just fine and the fishing experience more enjoyable.
In the shop we sell mostly 5 and 6wt rods and matching lines to novices, most of whom are initially targeting panfish, trout, and largemouth bass with nymphs, dries, buggers, and small bluegill poppers and foam bugs. Since largemouth bass is the most popular freshwater fish around here, most who stick with it eventually want to fish larger bass bugs. As much as we would like to sell them a 7wt or 8wt rod, more often than not we sell them just a 7wt or 8wt line after putting such a line on their rod or a similar model and having them cast it so they can try it out.