Here's what I learned on multi carbed bikes. First I want to say, that everybody, and I mean everybody on this site, has more talent and knowledge than I do. I learn from my mistakes and listening to talented people.
Vacuum synching can help hide issues. Don't get me wrong, vacuum synching is the goal to get your bike to it's optimum. But here's where it gets sticky. If there is a leak somewhere, synching can help hide it. You get an idle, you synch and your bike runs well, then you notice it idles funny when hot, or cold, or the idle changes every time you start the bike, it might fade on top end or bog on shift changes. It could lag off idle or flat spot at different RPM's.
So as bad as I fought it for years, the reality is, there are dozens of places your carb can leak. Slide rubbers, screw rubbers, intakes etc. but here's one almost everybody misses. The linkage that goes in the side of the carbs usually have gaskets and sometimes neopreme or plastic washers. I can say as truth, every time I've replaced them, the bikes ran better. Along with a rebuild (many carb kits don't come with these rubbers or washers) the bike then runs like new or better.
Once all the leaks are found, the synch brings everything to the balance the bike needs. I own a good synch, and for years I played Mr. synch for all my buddies. Everybody thought I was a genius, until I hung with some old dudes who put me to shame. They sat down and showed me the theory that doing things out of sequence will bite you in the ass. Synching is the goal, the finish line, but showed me on several occasions that the synch was hiding issues I didn't want to deal with. Who the hell wants to take the things all the way down, when I can hide the issue and go riding.
Lets get to the synch, but lets make sure everything else is proper to get there. I mean no disrespect to anybody, this is just something a hack like me has to do to keep from spending $ and time chasing ghosts...