I just had a quick look through some parts and it seems to me that the tradition is more 'origin at the highest surface with studs' than 'origin at part top'.
See for example:
40996 Brick 1x4 with Sloped Ends and Two Top Studs
50948 Slope Brick 3 x 4 x 0.667 Curved with 2 x 2 Cutout
50967 Slope Brick Curved 8 x 1 x 1 & 2/3 with Arch
41766 Slope Brick Curved 8 x 2 x 2
98284 Plate 2 x 2 Round with Hole and 4 Vertical Bars
4623 Plate 1 x 2 with Vertical Bar on Long Side and Long Arm
512 Electric Train 12V Level Crossing Sign
3218 Train Direction Switch - 4.5 Volt
4169 Train Signal Stand
And there are many more. 512 has a completely strange origin and orientation by the way. And with train parts like 3218 and 4169 having their origin at the top of the plate I would even argue that the 4.5V and 12V track assemblies are the parts with the inconsistent origin...
I would favour keeping the origin for 9V track parts at the top of the plate (sleeper).
I would have no problem with the little inconsistency it would cause. Despite our best efforts the catalog is full of them. For example the slopes mentioned above have their origin on top of the highest plate, with the top of the part another plate height above. These parts however have a very similar shape but have their origin at the top of the curve:
6005 Arch 1 x 3 x 2 with Curved Top
6091 Brick 2 x 1 x 1 & 1/3 with Curved Top
See for example:
40996 Brick 1x4 with Sloped Ends and Two Top Studs
50948 Slope Brick 3 x 4 x 0.667 Curved with 2 x 2 Cutout
50967 Slope Brick Curved 8 x 1 x 1 & 2/3 with Arch
41766 Slope Brick Curved 8 x 2 x 2
98284 Plate 2 x 2 Round with Hole and 4 Vertical Bars
4623 Plate 1 x 2 with Vertical Bar on Long Side and Long Arm
512 Electric Train 12V Level Crossing Sign
3218 Train Direction Switch - 4.5 Volt
4169 Train Signal Stand
And there are many more. 512 has a completely strange origin and orientation by the way. And with train parts like 3218 and 4169 having their origin at the top of the plate I would even argue that the 4.5V and 12V track assemblies are the parts with the inconsistent origin...
I would favour keeping the origin for 9V track parts at the top of the plate (sleeper).
I would have no problem with the little inconsistency it would cause. Despite our best efforts the catalog is full of them. For example the slopes mentioned above have their origin on top of the highest plate, with the top of the part another plate height above. These parts however have a very similar shape but have their origin at the top of the curve:
6005 Arch 1 x 3 x 2 with Curved Top
6091 Brick 2 x 1 x 1 & 1/3 with Curved Top