difference between additive dissolve and cross dissolve

Hi Everyone,

This is a dumb question. I have been editing for years and to be honest I don't use the additive dissolve that much. I went to the user manuel and it says that the additive dissolve: Adds the two clips so that the first clip fades out and the second fades in.
I have done some comparison tests. I still don't get it. I guess my point is that when I fade to black should I use the additive dissolve or cross dissolve? When I dissolve from video to video should I use the cross dissolve? When I edit to white should I use the color dissolve? When I edit still photos I use the cross dissolve.

Also I edit over many timelines and sometimes in my final product after it has been released to dvd it looks like some of the dissolves have a little too much noise in them. I have wondered if this is a compressor thing or if the dissolve is to long or maybe it is a case where I am editing across to many time lines. What is the proper way to edit with dissolves.

Thanks,

Steve