In summer I regularly water the foliage of junipers. It reduces spider mites, washes dust off and juniper can take up lots of water through the foliage.
In general, I use a shower attachment to a hose or watering can, and generously water the tree, foliage and area around it. The hotter and drier summer is, the more I ensure availability of water for the trees.
I would recommend taking advice from people that actually show they know what they are doing.