I have seen very few that "float" on springs. Stock Lobaugh are sprung fairly stiffly, and seem to run fine. I do have a few smaller models with brass adjusting screws, but they are so I can get axles out without pressing drivers off. And I have converted some to "sprung" when the original axle holes were not drilled properly. A Lobaugh Centipede tender comes to mind, as well as a Saginaw E6.
But my trackwork is the worst in the country, if not the world, and unsprung Northerns seem to do just fine. My 4-12-2 is set up for CLW springs, but so far is working fine with pads.
I agree - having everything parallel and not rocking on a glass plate is essential. Springs are just nice. Spring holes can be accurately drilled with a twist drill (Lobaugh) or an end mill (Loco Workshop). The spring doesn't know the difference.