No one offers complete coverage everywhere, and so you are best to ignore the commercials. Every region has local cell phone operators that give equal or better coverage than the national mega-telecom companies.
It is true that no one offers complete coverage everywhere, but one carrier (Verizon) is head and shoulders above the rest for everywhere I have toured or done other rural travel in flyover land. As far as "local cell phone operators that give equal or better coverage than the national mega-telecom companies", I have my doubt that they generally exist in much of the US. On the Trans America when we didn't have a Verizon signal we generally didn't have a signal at all. The one exception I noticed was the Riggins Idaho area and the carrier there was another one of "the national mega-telecom companies" (AT&T).
My highest recommendation is this: bring the cell phone you already have, as it will work for 911 calls anywhere. For all non-emergency calls, just ask a local. 99 times out of 100, people have been helpful and let me use their phone.
That or just use a carrier that offers decent coverage so you will seldom if ever need to ask someone to use their phone. I can't see myself asking to use someone's phone in a non emergency situation. I'd rather use the carrier that has the best coverage and call home only when I have a signal.
I never considered the ability to call 911 as a huge priority and if my phone didn't offer decent coverage for regular calls I'd leave it home. Maybe that is because I grew up and lived the majority of my life before cell phones or 911 existed. It might also be partly because a substantial portion of many of my trips were in places with no signal much of the time. Also even in the most remote parts of the US I usually see a car go by every 20 or 30 minutes. For an emergency call, I'd have no hesitation about flagging down a car.
Off road touring, I can see 911 being more useful, but most of the back country travel I have done there was no cellular signal most of the time any way.