I was told by a plumber a long time ago to make a habit of turning the gate valves off and back on once a month. Keeps them supple. And never open them fully, once turned a couple of turns, you'll get no more flow, so leave them there.
Which reminds me, I should exercise mine, as it is a some time since they were last done.
Yes, I've been told to do this too. But, damn it, I keep on forgetting and then I think "Well, it's been maybe 2 or 3 years since I last did it so maybe I should just leave it alone - on the "let sleeping dogs lie" principal". The only one that really worries me is the stopcock where the mains come into the house.
We had a new kitchen fitted by a friend who does John Lewis kitchens a few years back and after he'd finished we noticed, next day, that the mains stopcock was leaking in this way. As the whole kitchen was new I decided not to "fiddle" and called him back. Turns out he'd already tightened the gland on it when her turned the water back on so just as well I hadn't started "fiddling". The whole sink unit had to be stripped out to reach the compression fitting on the stopcock and a complete new tap fitted. Took him the best part of a day but at least he knew exactly how he'd fitted the units so no damage was done - and I found out how to switch off the stopcock out in the pavement - went right out and bought myself one of those long "T" handles, just in case!