I have been playing with this. Assuming that you have suitable replacements, you can remove some stock apps, although doing so may take away from the functionality of the device in some cases. For example, if you remove Browser.apk, you will no longer have the option to add a bookmark/URL shortcut to a home screen if you use Opera or Dolphin.
I have also safely removed extra keyboards and the following from Reborn:
Gallery3D.apk
Gmail.apk
Talk.apk
GoogleQuickSearchBox.apk
MagicSmokeWallpapers.apk
LiveWallpapers.apk
VisualizationWallpapers.apk
LiveWallpapersPicker.apk
I do not suggest messing with contacts related apks.
Again, be sure you have suitable replacements already installed if you remove any of these and still want the functions to work (e.g. an image viewer if you remove Gallery3D.apk). Your mileage may vary depending on the replacement you choose.