A soffit mount is intended to isolate the backwave (the soffit is effectively an extention of the speaker's front baffle), so it must be "flush" or "sealed". This will eliminate certain irregularities in bass response, but also has a broader effect on a speaker's sound that may (or may not) become an issue for any given listener. If you don't isolate the back wave, it's likely to be a mess. As an example, you could visit my den where Silverline Sonatinas are being mistreated by their in-cabinet location.
Good Luck
Marty
Good Luck
Marty