There are contradictory opinions. LOLA basic question, because I've heard contradictory things here.
My personal feelings, I believe the speaker should somehow be restricted from a fore-aft movement.
With heavy towers maybe not much of an issue but with lighter stand mounts, etc., Newtons third law states that for every action (force) in nature there is an equal and opposite reaction. As the speaker (woofer) cone moves forward and attempts to pressurize the room, it is also attempting to push the speaker backwards. Enough of this movement could modulate the FR and is not something to be desired. Opposing this undesirable movement can be controlled by weight (mass loading) or spikes, whatever.
Just my thoughts.
YMMV