There's no one right answer for how often or much you should water, because everyone's soil is different. Your ground might even be different from your neighbor's due to differences in amendments made.

You can't control the rain, but I think mulch does help. Even a little straw keeps my clay ground from sun-baking into a brick.

With my heavy, unamended clay soil, and lately with 100 degree temps and no rain at all I have been letting the drip irrigation run all night every five days. When the temps were in the 90's, I was running it about 4 hours once a week. My clay is great for holding water, but horrible about soaking it up unless I use the drip line to water. I have a separate small patch of tomato plants that I watered every day by spraying with a garden hose. They look so bad that I am going to pull them up.

My...um..."all-natural" method of letting some pigweed and johnson grass grow by the plants is paying off this year. They provide shade to protect from sun scald, which is bad this year. They also provide some handy soil tests. All I have to do it pull a weed and see if the weed's roots are dry :)

harveyhorses seems your method of determinting when to water have worked well so far ,,so why would you stop ? rain or not ?

A bit off topic, sorry about that:

@Cole_Robbie

I've got the same clay issues as you do, but with a twist - our May was terrible, rained non stop for 2 weeks and my toms developed shallow roots. Now we've got a heat wave, 15 consecutive days of 95 f. You can't imagine my watering horrors :) and just 3 weeks ago i had to pull 1 tomato cause it's roots have rotten.

Isn't it funny how the "right way" to garden is determined by the weather? Which, of course, none of us can predict, especially lately. If I knew it would be cold and wet, I'd plant in a ridge of soil and cover it with black plastic. But then the weather changes, and the 100 degree sunshine comes out to cook everything.

A large-scale tomato farmer near me puts out about 40 acres of tomatoes every year. This year he switched from black plastic to the kind that is white on top and does not get so hot. They have a single stake holding up each plant. I know they have a lot of sun scald this year.

My shallow-root tomatoes not only did not grow well, their taste quickly went downhill as the plants got worse. Sweetness went down, skins got tougher, and there was less flesh to the inside of the fruit.

My main concern was the need to fertilize and not over water, sooo not an issue three days later. They got their dose, and no rain, so wait for the sun to drop and give the poor things some H2O. They hold up in this heat a LOT better than I do, I wilt after about 45 min.
I guess I didn't add enough 'organic matter' last winter.