When it comes to water management in the garden, I have several rules: here are some of them, in no particular order.
Firstly, obviously, have as many water butts as you can, because the water is free, and is better for the plants than tap water. And did I say that it's free?!
Secondly, always make sure your water butts are installed correctly, that the connections are done up tight so they don't drip - dripping is wasteful, and it promotes the growth of unsightly moss and weeds.
Thirdly - not that we're going in any sort of order - have the butts where you need them. There's no point having six butts up by the house, if your veg bed is way, way down the end of the garden. If there are no convenient gutters down there, then you might have to be a bit creative with your water management - and a lot can be done with hidden hosepipes, siphons, and so on. More of that later...
Fourthly, although possibly secondly in terms of importance, always ensure butts are sufficiently high off the ground for you to get a watering can underneath them, and make sure there's a level area big enough for the can to stand on, while it fills.
In a perfect world, each water butt would have a dipping tank underneath the tap - that is, an open-topped tank or container which is big enough to dip the watering can in, for the fastest possible fill. This is also great for small birds and mammals to drink from, but to avoid heart-rending accidental drownings, either have a lid on it while you are not using it, or leave a narrow piece of flat wood angled into it, so that anything which falls can make their way safely out again along the “ladder”.
In my own garden I don't have room for a dipping tank, so I have two cans at each butt: this means that one can be filling, while I am using the other.
And that leads to the whole point of this brief article: buy enough watering cans that you can leave at least one, at each set of butts. I mean, what is the point of having a water butt without a watering can next to it? I always ask my Clients to buy more watering cans, if they don't have enough, and I ask them to get the sort whose handle does not go over the filling hole (and what idiot invented that style?) otherwise it's tricky to get all the water in the can, instead of over your feet.
And wherever I'm working, I always fill the watering can. It stops them blowing over in the wind, and when you are in a hurry, there it is, all ready to go, so you don't waste time standing over it, fuming, while it fills. Unlike a bucket or a bowl, they don't have any other function, you're not likely to need them for something else, so don't leave them sitting there, empty. Fill them!