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.
Thanks, I have a copy of the Selden instructions. I understand the stretch part, but if I up the size to say 1/4 inch, the stretch will probably be insignificant (stretch based on wire size and load applied), so then why not tension to only 267 lbs?