In my view fear is something that either is or is not present. If it is present then rather than supressing it by forcing a more centred state, I try to embody it and feel the sensation of fear. As Leo said I think that one can feel fear of many things, it may be of looking uncoordinated or clumsy, it may be of getting hit or hurt. I have found in my club that many people feel a fear of not being able to do a technique a certain way. This is certainly present in some beginners but also in many seniors such as myself who sometimes allow our fear of failure hamper our growth. I certainly become tense when I am fearful, but I find that if I observe the sensation of fear there is flow within this tension. It's something that I've begun to respect and work with.
On the point of spiritual growth and tension -IMHO I do not think that spiritual growth itself does not always lie in relaxation, even if the end may. The fact that I carry tension means that this is part of my being now. It is something that I must work with. Of course in Aikido practice there must be compromise, it is no good if one is so tense they literally cannot move. In this case it would be good to consciously breathe and invoke a centred response from the body. For this reason I find still meditation very helpful as I can embody sensations that literally give me temporary paralysis without having a tsuki hit me in the face