After a period of use the various bridle lines keeping the spar structure together may become slacker, meaning that some tuning needs to be performed on the kite.
The important thing to note is that the bridle lines on the back of the kite should be a little bit loose - all the tension is in the 'front' lines, especially the line connecting the silver to gold cap (that's CL-DL and CR-DR on my diagram). The kite should be symmetrical with the bowed spar close to, but not touching, the sail. You will notice that most lines are lacks headed through the end caps, knotted at one end and crimped at the other. Generally you won't be retying these. Two lines do however have knots - these are the ones you tension.
As long as the kite isn't twisted when you look at it at rest, and the spars aren't falling out (and of course the kite is still flying) then there is no need to change anything (the 'if it isn't broke, don't fix it' school of thought).
When I have experimented with this I will update this page with my experiences.
Something I have been told is that if you are flying in lighter winds then placing an additional bridle between the bottom stand-offs (FL and FR on my diagram) - under a little bit of tension to give a bit of slack in the sail - will improve it's light wind flying characteristics.
In all honesty I haven't tried this tip in all the years I have been flying a Deca.