How much should the temperature of irrigation water controlled?

Right back when I was first getting into gardening and doing research, I was told in no uncertain terms that watering container plants with ice cold water straight from the tap can cause 'shock', for want of a better word, to the plant such as my tomatoes for example. After some questioning, I was told that for plants like tomatoes, the ideal temperature of the water used to water or feed the plants should be about 18-23C, but that mixing cold water from the tap with some boiled water from the kettle (I don't have hot water from the tap in my house) could raise the temperature enough and that subjectively it should feel tepid. I've been using tepid water faithfully ever since, even though it is a pain having to constantly boil kettle after kettle of water to add meticulously to cold water from the tap.

At the garden centre and in other peoples' gardens and allotments, they don't seem to be so concerned with getting the water temperature right, and even use hose pipe sprayers connected straight to the mains, so that cold water comes through when watering the plant. So, I was curious - do I need to be warming up the water each time I water? Does it make a difference at all? If so, is it noticeable?

If I could simplify things I just use the cold water straight from the tap without any problems then it would make my life SO much easier!

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