Our marketing team has two different versions of Adobe Creative Suite (CS5 and CS5.5). We can save the CS5.5 down to an IDML document so users with CS5 can open/manipulate the file but it reduces the file size significantly. What I do not know is if this saving down process reduces the quality of the file?? I am relatively inexperienced with these programs as I am self-taught. Thanks for your help!
The major reason the file is smaller is that it contains no accumulated change info from editing and no image previews. If you used any features that were introduced in 5.5 you'll lose that, and there's a small chance you'll see some text reflow between 5.5 and 5.0.
IDML is good for repairing files.
But IDML is not good to be used as exchange in a workflow because every export of IDML can change things in an unexpected way. So I would strongly recommend that everyone in the department involved in the workflow uses the very same version, anything else will let you struggle in a later point of time. When I have to exchange files with a client I use always their versions and avoid IDML in exchanging files in any case possible.
Between CS5.0 and 5.5 are also several changes possible, because some bugs related to the text engine became fixed. E.g. in CS5 if you had an automatic TOC or cross reference in a text frame with 1 or 3 round corners and 3 or 1 normal corners, automatic number got shuffled in that frame, like page 345 became 543. When this was fixed some changes in the text engine have been done.