Another rule I routinely break! I've rarely replaced them on things like shocks/struts, anti-sway bars, etc. and have never had one back off. These days the neglect is not about money, just convenience and timing, but in the old days I reused the same ones many times on my 510s and 240Z because I was always pulling them apart to try different things.
But, if you can plan ahead and get them, it's worth a few bucks for peace of mind.
KGP, to what degree do you think liability concerns prompt them to suggest replacement? They certainly aren't noticeably easier to remove the second time.