The whole world does not end if the engines are not in perfect sync,,,,,,,BUT remember that if they are not, you do not have equal thrust.
Yes, I try for the "Harmonic", at least at full throttle.
Now, warning. NEVER Tweak the low engine up to the faster engine or the weaker to the more powerful. It is inviting disaster as an engine out. If one engine is more powerful than the other, set the weak engine and back off two or three clicks rich. Now match the strong engine slightly richer to match the other.......NEVER THE OTHER WAY AROUND!!!
Now, always use a partner. Hold the plane vertical and go to full power. If one sags, NEVER TAKE OFF! The engines must be reliable in this configuration.
If you want to get fancy and really want the Harmonic sound at most throttle settings, put each engine on separate channel and mix together to the throttle control. Now, using exponential, similar to what the helicopter guys do, you can match the acceleration curve of the engines. It does take a little more careful setting of the needle valves for consistant equality, but it does work.
Good Luck,
Twinman