Guys...and hoppy...
......remember..the plans themselves expand and contract slightly due to the nature of paper, and how it reacts to humidity/dryness in the climate. Some companies will lay claim to this, and some don't mention it. Being a printer myself though, I can tell you..it's fact. Ever slight sometimes, and sometimes it's very obvious. All paper plans are printed on is NOT created equally.
Whatever you do on one side of the wing for rib placement...do it to the other. It's a precisely cut kit, and the dimension of the plans will change well before the dimension of the pre-cut component. Your plane will end up fine, if you just make the right side same as left, regardless of the plans, do use them for a definate guideline though..for sure. Reduction, and enlargement occurs everyday in the printing world too, unbeknownst to some.
If your a line or so off on your wing lay-out, make a note as to the locations, and carry it on to the next half. Don't stress a component that's supposed to fit a certain way because it won't line up exactly on the plans.
Now...If your more then a printed line off(1/16" or so, give or take), then there could be a problem...but as far as exactness in pre-cut components to the plans...the plans can change slightly.
The change is usually more prevalent on longer lines, depending on the grain of the paper/plans. I do hope this helps