I have done the same thing as Longy mentioned...
and really haven't watered my tomatoes except
for when I initially planted them...
*and we have had our fair share of rain on and off, but I haven't watered them since except for rain, and my
plants are looking good...
I haven't gotten any tom's ready to pick yet,
but I will let you know on how it works for
me this year...
I have to say, IMHO, of
growing tomatoes,
alot is to watch your
plants/fruits, know your soil and your own water amounts, watch the weather reports ~constantly~ and
pick them accordingly...
I don't think there's any nutrient you can add to the soil to help prevent that stuff....
And like John said, maybe if your weather forcast calls for scattered showers all week
*Like we've gotten here all week so far*
and you have on'es that are close, and the color is close to pickin', pick them..
*also alot has to do with how an individual uses them.. if they pick one at at time for eating, or if they need alot for canning or such*