Unless if gets really foamy leave it alone. Its been said here the only reason people recommend adding any calcium is a carry over from pool mantenance. I don't think soft water is any harm to hot tub materials.
Gonna dissagree with the big kahuna here...
Soft water can be corrosive to the equipment and WILL cause foaming. It's an easy adjustment with a calcium hardness increaser. Don't wait for problems to start but instead, make the adjustment now.
Do you know what your hardness reading is?