Chainline adjustment

Is it a good idea to add a spacer between the BB shell and cartridge ring to push the chainset out a little, so that my chainline is more straight? Currently my chainline goes outside to in from the freewheel.

http://sheldonbrown.com/chainline.html#adjfront bullet point 3 mentions this. However I have a cartridge type BB so I'm not sure if its a good idea.

Yeah you can space a bb.
Velosolo make some.
CNC Threaded Cog Spacer Kit
http://www.velosolo.co.uk/shoptrack.html
The British bb should be the same diameter as a track cog with a british thread

Ta mate. Do they have to be threaded? I was thinking of buying one of these http://www.ebay.co.uk/itm/221571345316?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

The velosolo ones aren't threaded I aaid that to give you an idea on size. They come in a set of 5 or 8 with all different sizes.

No, they're not usually threaded. They are the same size, and often used as, freewheel spacers.

So I bought a 2.5mm spacer and installed it today, but it seems I'll need a few more. Is it safe to add about 7.5-10mm of spacers to one side of the BB?

The axle is one piece of metal so the more positive space you take on one side turns into negative space on the other, so the left crank might try to get inside your Bottom Bracket shell. Also, as it is a cartridge BB, the cup will have trouble going all the way in the other side and might not hold the cartridge properly. Obviously you probably want another BB with the correct length alex to keep everything in the right place.

Is it safe to add about 7.5-10mm of spacers to one side of the BB?

I don't know about safe, but if you need to space it that much (apart from the problems already mentioned, that you'll likely encounter), something else is wrong: what does the chainline measure at the rear?