I have flown aircraft where ashes have been dropped quite a few times. Fortunately, I was well briefed before the first event. The basic problem is always the in-elegant blow-back of somebody's loved one's ashes. In a C172 this can be solved by flying clean at about 65 knots and dropping them out of the left hand window. If you are using a PA18, a gentle side-slip to the right is your friend, but again use the left hand sliding window. Opening the double doors tend to frighten the distressed relative. In a glider, firstly allow the dropper to practice on the ground. Again fly slowly with a gentle right hand side-slip. Also, the urn that normally used for holding ashes looks better on a fireplace receptacle because in a glider it will probably result in a broken DV panel and/or canopy. If dropped, it might hurt someone. A small, plain plastic bag is the most useful device to hold the ashes to be dropped. Tape the bottom of it to the dropper's hand before flight.
As for a helicopter, I haven't a clue.
For the actual flying, brief them well but gently. Tell them that you will go somewhere where there is little R/T and that you will turn the radio/s off. Get up above the PPL's standard 2,000' and perform the HAZEL checks and give the "dropper" a few moments. Start the side-slip and then tell them that it will now be OK for them to drop the ashes. They will comply.
Lastly, ignore people who tell you that you can't do it. Legally, I'm sure it's wrong. But so what.
PM