Mixing your project in different ways will produce different results, and so the answer to whether you should mix your game in any particular format is dependant on what your design goals are. Even a 2D game can potentially use 3D sound to indicate the relative locations of in-game elements (many 2D games that use a top-down perspective do this) but in other games positional audio might be less desirable.
The biggest consideration when deciding a project’s speaker format is usually which platforms it targets, and what formats those platforms support. Most mobile platforms, for example, only support mono or stereo output; If you intend to release your game only on mobile platforms, mixing your project for surround is unlikely to provide much benefit.
Incidentally, project format is still a setting in 1.08, but it can now be set on a per-platform basis, specifically so that you can mix your project differently for different platforms. As such, to change the format for a particular platform, navigate to the “Build” tab of the “Properties” window, then select your target platform from the “Project Platforms:” list, then select the desired format for that platform from the “Speaker mode:” list.
Removing the 3D panner from your events will not affect the final output format of your project; That’s determined by the platform-specific format property described in the preceding paragraph. 3D panners do, however, set the speaker format at the point where they are positioned to your platform-specific speaker format, so they should not be removed if you intend your events to make use of 3D panning.