It is not a good idea to mount them flush. Here is the problem. The screen will reflect some of the sound waves back because at higher frequencies, it is a reflector. The front of the speaker is usually very reflective so the sound bounces back and forth creating comb filtering (notches taken out of the response of the speaker). It definitely changes the timbre of the speaker and is not correctable using EQ (since both the sound and its reflections always remain the same).

Screen manufacturers talk about high frequency loss but not this problem.

You can hear this effect rather easily. Take a piece of your screen material, have it stretched on a frame, and then have someone swing it in front of a speaker as you listen at your normal position. You will hear a distinct change and not for the better.

A good solution which you should follow even if you have a gap, is to tilt the speaker. You should do that anyway by pointing the speaker to the sweet spot (doing so means that the best sound of the speaker, the direct sound, is heard by the guy paying for it). By tilting it you are reduce the bouncing back and forth. You can hear the effectiveness of this in the above test.