Last time I was in Iowa (around the turn of the century) WBZ was a regular catch with a moderate signal. I don't think it's that way anymore with WHO's Iboc.
No, I was around Cedar Rapids and Dubuque last year and WBZ carried a decent nighttime signal into those areas. I think that WHO causes more trouble once you get into central Iowa.
The "38 states" thing is an old radio cliche that probably was true for some east coast stations back in the pre-TV days. WTIC had a nighttime host back in the 1980s who loved to use that line. And, it was never true for them thanks to the existence of KRLD. It would seem that very few stations could actually boast of actually having a listenable signal in 38 states. The ones that come to mind would be WLW, WGN, WWL, KMOX, WBBM, WJR, and WHAS.
Many others (WSCR, WLS) get disqualified thanks to Cuban/Mexican interference in the south. A Cuban blots out 670 in PA more than half of the time; I'm sure it's decimated by the time you get to SC or FL.
WBZ? It's pretty tough for any east coast station to pull that off these days. They'd have to get some signal into each of the plains states (including Texas) and all of the south (inc'l Florida and LA) which is pretty unlikely thanks to IBlOCk, Cuba and Mexico.