It really depends on how hot it gets.
In the summer I water around sunset, because the sun is so harsh and its get so hot here that if you water during the day very little water actually sticks around to be available for the plants because it evaporates so fast. On top of that especially is smaller dark plastic pots the soil can literally get very close to boiling, most plants do not like that.
Watering when the sun is mostly gone helps reduce evaporation and heating up of pots due to the sun hitting them actually using the water to give a bit of a cooling effect. However, once night time lows get really high - by July we generally do not go below 80F (~27 C) and if we have strong monsoon season in August we often have a few weeks when things do not cool down below 90F (~32 C) - I get more nervous when watering certain plants at sunset.
The problem is that at those temperatures many plants with CAM respiration have issues being wet and hot overnight - the high temperatures inhibit the reversal of their respiration and having water present when that happens is not a good thing - plants can almost literally explode. I have had Aloes turn to mush literally overnight. So if we have strong monsoon (not this year) when the night time lows get really high I may water very early in the morning (are lowest night time T is usually just before sun rise). I have already lost 5 Aloes this year and a couple of Agaves because of this - Once it happens a few times you learn to recognize the specific way things die...
To put these night time Ts in perspective our day time high has been above 110 (~43 C) for the last 37 days or so, we probably been above 105F (~40 C) since the second week of June... So watering time is definitely important here especially in summer.
The rest of the year when things are <100F during the day, I water earlier in the day, but I have not really found it to be that big of a deal if I do water close to sunset.
