Read Sheldon Brown on chainlines. Chainline on Bicycles with Derailers It is nice to have a perfect chainline but many bikes (probably millions) do not, usually through time, replacement of bottom brackets and cranks, square taper cranks sitting further inboard after many removals and installations and in some cases the installation of the bottom bracket. Some folks like chainlines further in to have better chain angles in low gears. Some ot bring their pedals closer together for a lower "Q-factor". Etc., etc.

Now if you want to pass through the hallowed gates into the heaven of perfect bikes, your bike will have to have the 43.5 mm +/- 0.3 mm chainline offset from centerline. That way, when you install one of those hallowed wheels, the angels can sight up that middle cog to exactly between your chainrings. (Of course, with two chainrings and an odd number of cogs, or and odd number of chainrings and an even number of cogs, a straight chain never happens. But if we don't want to jeopardize our place in bicycle heaven, we keep our mouth shut.)

Sight up the middle cog to the chainrings. If they line up to within a half cog, the bike will do just fine. If it's off by that little, ignore it until you have reason to mess with the crank/BB or wheel anyway. More than that and you might want to take action or maybe just bear in mind that some of you big crossover gears will be hard on the chain, cog and more work for the rear derailleur.

(This is a lot like wheel alignment in cars. Yeah, you want all 4 wheels parallel and parallel to the car frame, but look at how many cars out there happily crap down the road sideways.)

Ben