Converting SS to fixed - Will the chainline still be right?

1. Normally a flip-flop hob will give the same chainline both sides (otherwise there is nonpoint to the beast, because you can't turn your wheel on the road without respacing) but single freewheels don't necessarily have the same width as fixed sprockets (and the concept works best fixed/fixed)
2. I used to think the same as Stickman but now I have come to the conclusion that it does matter having the chainline as close to right as possible, to cut mechanical losses. However it makes more difference to the running of a fixed wheel than to a freewheel, because you can generally run the chain a tiny bit slacker on a freewheel, and because where you notice the error on a fixed wheel is going between forward pedalling and holding back (which you don't have with a single freewheel)
3. Measuring. Start with your hub. Measure the distance from washer or locknut that contacts the fork-end to the shoulder of the sprocket thread (the part that the sprocket threads up against). It should be the same both sides. Note this dimension for each side on a piece of paper. Now measure the dimension from the contact face to the teeth of your fixed sprocket (outside to outside or outside to inside, doesn't matter but note which because you want to measure this on your freewheel). Measure the same dimension on your freewheel (remembering that the adjustable cone, normally with two dimples for undoing and with maker's name or similar goes to the outside so you want to measure the other side).
4; If everything is the same, nothing to worry about. If not return to your measurements and do the subtractions. If you have less than 2mm difference I wouldn't worry. If the error is 2mm and over go to the site of Velosolo Bikes (sorry I don't have the link on this computer) where you will find various spacers for going behind sprockets or chainring bolts, depending in which direction you need to correct. Spacers may well be available elsewhere but I am ppretty certain that Velosolo will have what you want.

Thinking about this you shouldn't need to respace your chainrings because otherwise you would have to respace every time you turned your wheel. Measure to be certain. I have a fixed sprocket one side and a freewheel the other on my bike and I don't have any worries when turning the wheel in the middle of a ride. I can always check my wheel to see exactly what it gives. Normally everything works around a chainline of 42mm from the centreline of the bike.

That is a page of Sheldon's site I have never looked at. Thanks for the information. Don't forget, if you need to run a slight error, it is better to have the fixed right and the error with the freewheel - but your flip-flop hub is probably designed to be useable both ways round.

Go for it
What's your hub, using mtb spacing like that? I didn't know that there were any mtb specific flip-flop hubs; I thought they were all ss conversions on standard cassette hubs (with Velosolo discmount sprockets for the fixed side). You live and learn.
Of course, unless you need to alter the length of your chain (or you run with only one brake) there's nothing to stop you flipping the wheel whenever you want, even in the middle of a ride.(Best avoided if you are racing though).
Don't do anything to your chainline, it will be just fine as it is.