Angled top bars are the traditional method for building frames. It predates foundation, and was used by Langstroth, A.I. Root and others. The reason for it is that it works.
"HOW TO SECURE STRAIGHT COMBS. "The full advantages of the movable comb principle is only secured by getting all the combs built true within the frames. Upon the first introduction of movable frames, bee-keepers frequently failed in this although much care and attention were given. Mr. Langstroth, for a time, used for guides strips of comb attached to the under side of the top bar of the frame. This is a very good practice when the comb can be had, as it usually secures the object besides giving the bees a start with worker comb. Next followed the triangular comb guide consisting of a triangular piece of wood tacked to the under side of the top bar, leaving a sharp corner projecting downwards. This is a valuable aid and is now universally adopted." --FACTS IN BEE KEEPING by N.H. and H.A. King 1864, pg. 97
"Improved Comb Bar.--Mr. Woodbury says that this little contrivance has proved very effectual in securing straight combs when guide combs are not obtainable. The lower angles are rounded off whilst a central rib is added of about 1/8 of an inch in breadth and depth. This central rib extends to within 1/2 an inch of each end, where it is removed in order to admit of the bar fitting into the usual notch. All that is necessary to insure the regular formation of combs is, to coat the underneath surface of the central rib with melted wax. Mr. Woodbury further says, "my practice is to use plain bars, whenever guide-combs are attainable, as these can be attached with much greater facility to a plain than to a ribbed bar; but whenever I put in a bar without comb, I always use one of the improved ones. By this method , crooked and irregular combs are altogether unknown in my apiary." Most of our bars are made with the ridge; but should any of our customers prefer the flat ones, we keep a few to supply their requirements"--Alfred Neighbour, The apiary, or, Bees, bee hives, and bee culture pg 39
"Top bars have been made by some hive manufacturers from one-fourth-inch to three-eights-inch strips, strengthened somewhat by a very thin strip placed edgewise on the underside as a comb guide; but such bars are much too light and will sag when filled with honey or with brood and honey..."--Frank Benton, The honey bee: a manual of instruction in apiculture pg 42
"Comb Guide.--Generally a wooden edge, or a strip of comb or fdn., in the top of a frame or box, on which comb is to be built...As the comb guide is 9-16, and the cut in the end bar 3/4, we have 3-16 left for whole wood in the top bar, as at A, and the table should be set, as to leave just this amount of wood uncut. Even if the fdn. is fastened in the frames with melted wax as many do, I would have such a comb guide, because it adds so much to the strength of the frame, and obviates the necessity of having a very heavy top bar. The bees will, in time, build their combs right over such a comb guide, and use the cells above the brood for honey."-- A.I. Root, ABC of Bee Culture 1879 edition pg 251
"A comb guide proper is a sharp edge or corner in the frame, from which the comb is to depend, the bees usually choosing to follow this edge, rather than diverge to an even surface; portions of comb are sometimes used for the same purpose."--J.S. Harbison, The bee-keeper's directory, footnote at the bottom of page 280 and 281