they should be as close as possible for best imaging and sound. Ideally coaxes are the best arrangement, however few companies put their best speakers in this arrangement. Great distances between the two can also lead to phasing problems and other sonic nuances.
As a practical matter if you don't care about staging, imaging and all that, put them where you want. The casual listening probably won't notice the difference if they don't know what the are listening for.
As a geewiz, FYI, kickpanel placement is very popular for imaging in pro audio cars as it gives the most equal distance possible between left and right sides. Using the balance knob isn't quite the same thing
