This is how my Opus works (Pro 4.50)
If you hold down the
CONTROL Key and start drawing with the straight line tool you get lines that only go horizontally and vertically. It doesn't matter if the grid is on or off.
If you turn on the grid the line will snap exactly to the grid dots. If the grid is on but you hold the SHIFT key nothing will snap and you can draw lines in any direction you want.
With grid off and SHIFT pressed the line snaps but I'm not sure if it really snaps to the gid dots since they cannot be seen.
Hope this helps