Depends on the coin you're mining.  If you set it up to allocate 2680 MB or so (just guessing what it might be), you'll be good at lookup-gap = 2 until N=12.  You'll have to fiddle with LG at that point.  Anytime N changes though you want to fiddle with your settings - it's not always obvious what the best settings will be.  Sometimes running a lower lookup gap and limiting the intensity (using --rawintensity) gives better performance than going with a higher LG and using all the shaders.

For YACoin at N=14, My R7 250 GB card started at 1.6 KH/sec using the OLD yacminer.  After I updated the software and figured out how to configure it properly, I found settings that gave me 2.5 KH/sec.  I ran with that for a month before experimenting with lowering the LG by 1 and limiting the shaders.  I got an extra 8% hashrate by doing this.  I will eventually be releasing a spreadsheet or web application for y'all to put your settings into to get the same kinds of results, but it won't be until the algorithms are integrated into YACMiner.  I still have one performance metric to come up with a formula for, but I've got quite a bit of data for it now.