I always thought you were supposed to sheet in the Genny.
I tried to sheet in the Genny so as not to deal with the leech flapping around, but when I did so, I lost a lot of boat speed.
If you are talking about sheeting in the genny when it is behind the main and the wind is on your quarter or aft - no you don't want to sheet in your genny. Let it fill. Next time you are doing this, try bringing your boom in to the centreline of the boat to get the main out of the way, then let your genny out until it's flapping and pocketing. Now move your genoa lead car forward a bit - I have no idea what your boat is like so can't say how many stops but you want it forward of where it is when you're beating, but not too far ... better to err aft than forward...
Now start hauling in the genny until it fills nicely and is giving you a decent pull on the course you want to go. If the leech is still fluttering and it bothers you, then play with the leach line - but remember that you're going to have to loosen it when you're on the wind again...unless you're racing it's easier to turn up the stereo
When you have your genny set, let the traveller out a bit and then ease your mainsheet out so that you have a bit of twist/cupping in the main. It will lift the boom up so that the bottom of the genny is not so shortwinded...
If you want to get really chronic about it, you can make a note about what angle the apparent wind is, and then mrk your genny track and your lines with a marker to give yourself a reference... Play with your sheets and your genny car while you watch the log to see exactly what trim works on what point of sail... Enjoy !