the risk is small, but it's there. a diesel runs on higher compression, and you'd think some water wouldnt hurt. i've seen several diesels throw a rod or blow the top off the power assembly, from a leaking water jacket. (emd locomotives) i've also hydro-locked a diesel, trying to crank it, with water in the cylinder. bent a rod, but didnt break it.

i use an empty dish detergent bottle, and just drizzle it, in a really light stream, with the motor at a very high idle. maybe 2000-2500 rpm. you'll know you're adding too much, when the motor starts to bog down, as it fights to compress the water. i also have the car hot from driving before doing this, or you might fill the mufflers with water and crap, helping to rust/clog it.

the gas today is a lot cleaner (some of it's clear, like water, vs the dark orange stuff we used to pump) and as long as your guides arent leaking, and you dont just idle the motor endlessly, it shouldnt be all that dirty. once or twice a year should be fine, if you worry about deposits. if the plugs are clean and dry, i would imagine the rest of the combustion chamber is, as well.