Neil - Not sure what you mean by "positively locate" your top bars. If you mean to secure them in place, then no, not really. The bees will glue them in pretty well, which you'll need to break free to do inspections, manipulations, etc.
With respect to your other question, there's a wide range of opinions about how to best prepare the bars for the bees to draw proper comb. While some use nothing at all, most agree that some sort of guide is important to have. Choices are many, from melting beeswax onto a string stretched down the center of the bar, to cutting a groove down the bar and filling that with any of melted wax, Popsicle sticks, wax foundation, etc., to milling the bar itself so that it tapers down to a point (sort of like an upside down triangle when viewed from the end). I took length of cove moulding and fixed them to the bars. I'll see how well that works in about a month when my package moves in.