I'd always try to lock my bike at a control if it were out of sight etc. But with a lightweight deterrent lock. I'd bring my bike in at the end at a village-hall-in-town finish as well, if I could (eg Bryan Chapman finish) but not worry at somewhere like Kings YH at the overnight stop, for obvious reasons.
It is all about risk and chance; AUK saw over 19,000 rides completed last year so mostly audax riders don't seem to face such bad odds. But it happens - a bike was stolen from an intermediate control on a 200 a few years ago.
+1
Even on clubruns, not everyone locks their bike at the cafe.
I always carry a cheapo lock. Just replaced one that I lost as it happens. I'm always loosing bike locks.
My recent investment in cycle security is a Tesco Value lock. not very robust and easily cut with a hack saw. But very lightweight and a deterrent.
I locked my bike at every control for this event, as I usualy do.
However, I didn't at the finish. I left it in sight of the organiser (not that I expected him to be responsible for it, but it's only visible to passers by and it's not a throughfare. I did consider locking it, but as I was stopping briefly, there were people about and it was an 83" fixed gear bike, i reckon the chances of it being pinched were very slim. plus i sat down where I could see it\ and did keepp an eye on it.
I do sometimes get comments that I'm bothering to lock my bike up, but I always say that it's a long walk home for me and it's bad enough cycling this far. It only takes a minute to lock up my bike.
I never bother with insurance. In over 20 years of cycling, I've yet to have a bike pinched, so I can afford to have my titanium ATB stolen, but I'd much rather keep it thanks.