This is something I've heard and don't know alot about it but it may be something you might want to think about before you set up a drip type watering system.
I've been told that if you water trees too much they'll develop a large majority of their roots near the surface and when you quit your watering it can be difficult for them if there is a drought. In comparison, if you water them heavy once in a while they'll develop there roots in a more natural way and there roots will develop deeper into the ground to reach the water below. Like I said, it's something I've been told and don't know how much truth is to it, but it does make sense.
I planted 11 apple trees this spring and had to water them about once every 10 days for the first month to make sure they were going to be ok, but lately we've gotten so much rain I haven't watered them at all and I don't think I'll need to again this season unless the ground gets very dry. I've been digging down near the tree and the soil is moist an inch or two below the surface and I'm assuming that's plenty good to prevent them from drying out.