The tension is important because wire does stretch. If the initial tension is too low, the leeward shrouds will be flapping around. One way of tensioning is to alternately tighten the leeward shrouds on different tacks while on a beam reach until they are both just slightly loose and the mast is straight at rest. There was recently a pretty good thread on this with links to Selden Mast. They have a page on the topic.
This is the correct method of rig tensioning. After done take measurements and use these measurements for next rig tensioning. 15 % is just about right and might change. Considering the length of lowers one can easily decide that the lowers should be tensioned less than the others.