It might be; I'm not entirely sure, though. From what I've read on here, 630 rpm seems to be the factory setting (or around there)
I think 550 is acceptable if it was set to be there, but I have the ELR knob set at position 4, which is factory setting, so I think it should be higher.

I'd also like to mention that the surging / shudder part seems to go away after the engine has been at operating temp for a while; or at least, is not as apparent (For example, after a 15-20 minutes drive at 80+C)
Idle remains low though.

__________________
Dan
2005 E320 CDI - 246k
1987 300SDL TD05-16g, Herlevi pump, Elbe manifold, 2.47 LSD - 213k
Past: 1987 300D - 264k