Here's your 'technical' answer: Sails are made from polymer materials which are 'plastic'.
The definition of 'plastic' is that which constantly deforms or changes shape over time under constant load (tension, etc.)
If you keep your (plastic) sails always tensioned you will ultimately permanently DEFORM them. If you dont want to continually replace 'stretched-out' and permanently distorted sails .... unload them if tensioned when done sailing.
That said, a taped-luff sail (continuous luff support tape that fits into a furler
foiil) isnt 'set up' in its foil with 'much' tension by most 'cruisers', .... and if you stretch out the luff with 'heavy tension' for 'shaping' purposes most inexpensive furlers
will jam or bind anyway ... so the 'usual' set up on most jib
will need little luff tension anyway and therefore wont make much difference.
However for those (few) who actually 'shape' their sails, if you leave the halyard or outhaul, etc. TIGHT for long periods of time when your not sailing the boat .... those edge dimensions and the shape of the sail is going to 'radically' change by permanent 'plastic' deformation.
Sailmakers LOVE folks who dont 'unload' sails, ... creates lot of business to replace 'blown out' sails.
Best practice is to UNLOAD the sails when not sailing ... your sails will 'live longer'.
BTW - PDQ has it correct about the torlon or other 'plastic' roller bearing 'balls' in furlers
.... they're 'plastic' and even just because they are an engineered 'plastic' that resists heavy applied stress - they are still 'plastic' .... UNLOAD 'plastics' if you want to keep them 'undeformed'.