Nothing in the N900 should wiggle at all - if it does, don't make it worse!

But when plugged in, USB connectors have always had a small amount of movement. As long as it's only the cable moving, slightly, with some force needed to make it move, that's ok.

I still would not wiggle the cable for fun. That'll strain the connector inside the unit; even a slight oscillating strain tends to grow microcracks until something breaks (same mechanism as metal fatigue).

To insert the cable I always line it up, then use the minimum force, smoothly and carefully. To remove it, I always pull it out in a straight line away from the unit to equalise the stress across the internal contacts, applying a very slight slow wiggle which reduces the force needed, and with my fingers on the connector also touching the case, to ease it out with control.

I don't take so much care with any other device! It's these reports of a bad internal mounting design, on such a valuable device where even sending it away for repair would be a great inconvenience, perhaps making me paranoid.