I used to save them for Christmas, then last December my Nectar account was hacked (I never did find out exactly what happened, Nectar wouldn't tell me.) The £50 I had saved was spent in an Argos store. Nectar did eventually refund the points to my account along with £10 of points for the inconvenience.
If you google it, Nectar scams are rife, with the points commonly being spent in Argos. Since then, as soon as I have £2.50 worth of points I redeem them off my shopping bill.
I use mine at the ' double up points week' in November. Last November I bought 2 soup makers for our lads. There was a problem with the nectar card and Sainsburys staff phoned Nectar. Afterwards I noticed instead of points being taken off they were added. Have tried in Sainsburys 3 × to sort this but still have far more points than I should have!