The most free-rolling trucks in HO were (and I believe still are) the Lindberg trucks. They would roll on a about 0.5% grade, even on a lightweight car. Therefore, I use 0.5% (1/2" in 8ft, or 1/16" in one foot!) as my maximum spur and yard grade as part of my layout design standards.
Testing as I'm building using levels and straight edges is critical to ensuring actually meeting the standard. Vertical height precision of the rail tops really needs to be better than 1/32". Cookie cutter subroadbed and risers are easier to adjust than flat top layouts. When hand-laying track, sanding the tie tops before spiking rail is the final adjustment. If using cork roadbed and flex and/or sectional track, sanding the top of the cork smooth is just as important to getting things right. A straight edge used vertically is the final check for vertical smoothness, and used in conjuction with a level, the final grade check. The great thing about using all this care during track construction is that a lot of potential derailments are eliminated from the get go.
just my experiences