The ever lasting question.
I am from and in Wyoming, AKA the "Cowboy State". On almost any given day you can walk into a local restaurant in any small town, even our biggest ones here, and see one or more folks with their spurs on. You will also see there is a wide variation of the of the buckles and location. There is no right or wrong. It is a personal preference of the owner and also up to the design of the maker. Some are on the outside, some are on the inside. A lot of them are more on top, to one side or the other of the instep. Its up to you, as the maker where you want to accent the strap, (Put the Bling). Do you want to showoff your carving? Buckle on the inside. Do you want to use a sparkly big buckle? Then its on the outside. How about a fancy Concho and sparkly buckle? Inside or top. What ever you, or more importantly the customer wants.
Unless is for a US Calvary soldier or reenactor. As per US Army regulations - "The strap will be fastened over the instep of the footgear in such a manner that the buckles face the outside of the boot."