People wonder if we're going to update our best fit code page mappings, or even our code page mappings. The answer is no. Changing character mappings causes difficulties for applications and our experience has been that doing so breaks as much as it "fixes". We'd prefer applications move to Unicode, then you don't have to worry about best-fit, or if a character is supported.
Best fit behavior is the behavior of some code pages to map unknown unicode characters to a character that someone thought was similar that the code page supported. Examples would be mapping ｋ(U+FF4B, full width k) to k, or ĩ (U+0-129 latin letter small i with tilde) to i,or ∞ (U+221e, infinity symbol) to 8. Some of these seem reasonable, however we aren’t consistent in our mappings, most break the meaning, and some mappings (∞->8) changes the meaning completely.
The best fit mappings were created “a long time ago”, contained “omissions”, and haven’t been updated to include new Unicode characters. “Newer” code pages don’t necessarily include the same best fit mappings, and, by now, the mappings are fairly inconsistent and incomplete. So we don’t recommend that the mappings be used, and we don’t intend to change or “fix” the best fit behaviors.
We also don’t like to change other code page data either. “unassigned” code points can have arbitrary behavior or map to Unicode PUA code points. Some applications use those code points (perhaps unwisely) as formatting codes or to cause special behavior. Adding a mapping could break such an application. Other applications or systems may provide a glyph for an unassigned code point that round trips, however that might not be the designed intent, and changing the code point behavior could break those applications or fonts.
Code page standards are also sometimes extended, modified, or corrected. Changing the behavior however impacts all applications using that behavior and our experience is that such changes across the installed windows code base causes as much trouble as it solves.
So we like to keep the code page mappings stable. My recommendations for code page use are:
Hope that helps, more posts about common code page concerns are at http://blogs.msdn.com/shawnste/pages/code-pages-unicode-encodings.aspx