It can be common practice here between MO, AR, and OK. The reason is the lack of properties in the three states where they meet. But normally, I would be able to find sales of conventional houses, land, or farms within the state. But Poultry farms, dairies, large ranches, etc. do compete across state lines. Same with specialized property like churches, mega warehouses, small airport hangars, etc. But for residential property? It would have to be a very unique property before I would not confine myself to the same state...if close enough to a county line within the state, then perhaps I'd view it differently. Since some of our school districts cross county lines, I would, in particular view them favorably.
I always wondered how the heck you would appraise property in that narrow panhandle of N. West Virginia though... Go to Ohio? or PA?