Claudio, it is not correct to suspend the woofer for measuring the impedance. You will get a slightly different answer compared to when rigidly held. Additionally, depending on the cone mass and suspension compliance you will get a different answer with the driver horizontal versus vertical, due to the suspension sagging under the action of gravity on the cone mass. This changes the position of the coil in the gap, changing Bl.
Being even more picky, the driver should be mounted on a large baffle, so that the air load mass on the driver is closer to the value that would exist when mounted in an enclosure.
Now of course this is all being pretty anal, because many measurement/analysis systems out there are not accurate or consistent enough to reliably assertain the differences.
Also, Thiele/Small parameters are not an accurate enough representation of the true low frequency performance for these small differences to matter anyway.