You can also use a battery to center the voice coil -- not leaving it on for a long time, but touching contacts to the battery to make the driver leap up and then settle down. A 1.5 volt battery won't hurt a woofer. Or even better, run a low signal low frequency (30 Hz or so) sine wave through while aligning. You can use clothespins to hold the outer suspension down while the glue dries.
I find gently tapping the cone all around the outer periphery gives a very clear indication of the alignment. If there are problems, you'll hear a clanking. So move the cone in the opposite direction.
