I haven't needed the stopcasting in macros yet. I use clerics for my healing and either have them in heal mode or dps mode with a toggle button. (I had a fancy setup but found that just sending Shift+1 for DPS and Shift+2 for healing to my cleric to change the main hotbar buttons works the best.)

cichard's idea of using the button on press and release is a good idea but I also find that my buttons don't get pushed 100% of the time with that feature. It probably is ping related, I am in Australia and the ping is pretty bad. I had this setup for the Sentinel's group heal and the ability that makes it instant cast in the same macro. Unfortunately it didn't work 100% of the time with the "on press and release" option in ISboxer.