Loads of people will authoritatively say one cannot accurately center the cones without first cutting away the dust covers (center circle of paper/cloth that keeps dust away from the voice coils), so that paper/plastic shims can be inserted around the voice coils to keep the cones centered.

Others will say that the stiff spider is enough to keep them centered, especially on the smaller sizes. And testing them before the glue sets permanently is impossible when shimmed, and trivial when not shimmed.

I fretted over this for a while, but in the end just did it without shimming, and then wondered what all of that fuss was about. It was a non-issue.

EDIT: The videos by Simply Speakers were the best IMHO.

Cheers!