I assume with spider you meant the plastic foamy retainer in the HD6** series ?
In that case I would say there has to be an open space behind the spider otherwise the driver would be over-damped and bass be rolled off and most likely linearity would be out the door.
When the question is whether the headphone is 'open' behind the driver then this depends on many factors in the design. There is fully open without acoustical feedback from rear to front, there is fully open with acoustical feedback from the rear. There is 'half open' and there is 'closed' but this can be in several degrees. Then there is the D8000 which is open but the space behind the actual driver is closed by a membrane.
When you mean the hole behind the driver magnet then the answer is: That depends on the diameter of the hole, the damping of the cone, the amount of damping that is needed and the application of the driver.
Or did you mean the space between the (often yellow) spider for aligning the cone/voicecoil of speakers to the gap of a loudspeaker ?