OK, here is my suggested logic and the reasoning behind it, which I hope is not overly verbose...
Default Explorer dialog start at logic...
List of explorer calls:
Contact Sheet Based (use last used - in the current app instance - if known, else folder of 1st image in contact sheet)
- Copy selected images
- Save As...
- extract JPG from Raw
- Export
- Edit|Load Selections/Save Selections (Now defaults to a common saved folder location as the 3 "Others" below) .TXT File
Others (Each with a separate remembered folder location that survives the app close)
- Edit|Load Structured Keywords/Save Structured Keywords. .TXT File
- Edit/Set Code Replacements (Add button) .TXT file
- Import/Export Preferences (I would think this is very low priority; but that's just me) .XPM File
I personally care most about the Copy, save as and extract operations because that is what I use the most. Per your instructions I included every operation I could find that results in an explorer dialog, and what I thought should be done.
Two of the three "Others", plus the Edit/Load Selections operation now share a common retained default folder yet all are by default *.TXT files, but have different uses and formats. I think this leads to confusion because the file picking dialogs list all these things in, presumably, the common default last used folder preference, unless the user is bouncing around with each save/load.
In the case of Edit|Load/Save Selections, I think it makes sense to use the same logic as the other contact sheet based operations because the contents are specific to a folder or set of remembered folders. Putting them all in one folder forces the user to pick one that's right. This is my theory; I have not actually used this feature except to write this up. Maybe input from other actual users of this feature would be helpful.
I think the 3 "Others" listed above should all have their own remembered folders to keep the various *.TXT files separate. You now use a single folder location retained between aapp closes. Personally I am going to create a "Photomechanic Settings" folder in My Documents or My Pictures, and then a subfolder for each operation. Again, I have not seen am immediate need for these features so my thinking is limited to a theoretical level. A better solution might be to define your own custom file types, maybe all starting with PM, one for each feature/format. At this late date, you probably don't want to do that for legasy reasons if nothing else, so establishing separate default remembered locations for each type is probably the best solution. As the app evolves, this issue of multiple structured .TXT file formats may increase as new features are added.
Basic Logic for Contact Sheet based operations (same as current Copy Images logic, I think):
1. Remember last used folder during the current app instance, separately for each operation, as in current Copy and Save As.operation.
2. Fist time after each app open, start at folder of 1st image in contact sheet (if the operation is contact sheet based)
Should the app remember folders after App close (registry entries)? I think this is subject to each user's preference (pun intended) and folder structure. Capture NX does this and I find that annoying because it always seems to take me to the wrong folder. As I recall, Adobe CS's browser starts off at the current open folder, as I suggest here, which I find intuitively easier to deal with and works better with my own workflow. It would make a good user preference but that then leads you down a path where users want separate user preferences for each operation- copy, save as, etc. For example, with my own workflow copy, saveas and extract makes sense as above, but export may not because some people may want their exported HTML in a completely separate folder tree, rather than intermingled in their image folder trees.