Moving from stock ROM to custom ROM usually requires a wipe or a factory reset. Otherwise, you will encounter force closes (FCs), heat issues, and possibly other problems. In short, headaches.
Even upgrading from a CyanogenMod (CM) to a major CM version will require a wipe, especially if it contains a major Android update. This is because the files / directories are now different (layout-wise, etc). I'm sure there are other reasons too.
Most, if not all, will also recommend you to do a wipe. This is the best way to ensure that the installation will be smooth.
Of course, you can still rely on Google Sync to ensure that your contacts (and other syncable data) can be restored.