There are many dialogs in illustrator that have the preview checkbox. I'm wondering if there's a way to make it checked by default. I think it's annoying to check it everytime I need it (that's 99% of time).
I assume your are referring to the preview checkbox from effects such as drop shadow.
Am quite certain there is no method. It was decided in a past Adobe Beta to keep those previews off as many of the effects can dramatically slow down a computer, and the effects are more likley to crash your computer if the preview is on.