Sprint customer here for over 10 years, signal strength/Roaming has not been somthing I have had to consider when making phone calls the last few years. When I need to make a call I do and it works. and I run coast to coast.
Don't forget, your physical phone has a bit more to contribute to service strength than people give them credit for. I have Sprint unlimited everything, but the Samsung Galaxy S II has some antenna issues. I've used the Blackberry Curve and Galaxy S on the same line in the past and they get 5x better signals, but no 4g support. Know the phone's antenna reputation.