I'm building up a fixed-gear with one of the largest frames I've ever seen. Which is another story. With geared bikes, obviously the chain isn't running straight in a lot of gears - if the middle chainring of a triple crank is lined up with the middle of the 9-speed cassette, I'll use all but the smallest and largest cogs with the middle chainring. With a fixed gear bike, chainline shouldn't come off at that much of an angle (middle chainring to 2nd-from-largest cog). But how far off does it have to be for things to start being problematic? Obviously there's some play allowed - Rivendell sells their Quickbeam which has two chainrings on the front, and presumably the cog on the rear wheel lines up directly between them. So that's +/- 2.5 mm of chainline at least, assuming the chainrings are 5mm apart. Are there any general rules of "how much chainline dissonance is too much"?
Thanks.

there is an article on Sheldon Brown's website on chainline. Check it out.

Also, there have been many threads on chainline in the SS/FG forum- search if you haven't done so already.

EDIT: I realized I didn't actually answer your question on how much is too much, as I don't know.

Oh, don't worry, I've read over all of Sheldon's articles, and I've done searches. I'm just curious what the practical limits of imperfection for chainline is, because obviously it doesn't need to be perfect to the millimeter.
An example: how much sub-ideal is a crank with 40mm chainline, used with a hub+sprocket at 43mm chainline?

the world won't end and your bicycle will take you places

It is 3mm sub-ideal.

In any case, Bill Ron says anything within 5mm of dead-on is fine, and I would take his advice over pretty much anyone here.

3 mm, I dont think that would be too much really

anymore than 7-10mm and Id be worried, moving the chainring fro outside ot inside or vice a versa and/or using spacers can fix small chainline issues

Word. There's way too many variables (chain, cog, and chainring for starters) to lay down absolutes. If the chain still turns the wheel, and you don't drop the chain going over the smallest bump, you're probably all set. Fine tuning will only make it quieter/wear less.

on a side note, this morning was my first ride on a bike that was SILENT, and damn, man, was it sweet. I tightened down my crank bolts, cleaned my sprocket and my cog, greased my chain with tri-flow (?), and spent about 15 minutes messing around to get the right amount of slack. Mmmm. I could heare the tyres, I could.

exactly what i was going to bring up. only i couldn't remember his name.