I think you can get away with either. But consider these points:
In the past drawing 2D and 3D together has caused problems on some cards with some drivers. This usually resulted in massive slowdown. This may get worse in the future as support for 2D is steadily dropped.
Most text doesn't change every frame and so you can leave most 3D mesh text alone once created.
3D mesh text often looks better because you have full alpha support.
3D mesh text can be scaled (useful for resolution independent HUDs) and rotated.
2D text will probably draw faster when everything works correctly (see first point).
In the past drawing 2D and 3D together has caused problems on some cards with some drivers. This usually resulted in massive slowdown. This may get worse in the future as support for 2D is steadily dropped.
Most text doesn't change every frame and so you can leave most 3D mesh text alone once created.
3D mesh text often looks better because you have full alpha support.
3D mesh text can be scaled (useful for resolution independent HUDs) and rotated.
2D text will probably draw faster when everything works correctly (see first point).