Wondering when I should stop using fertilizer for my potted fig trees.  One just started having rip figs, but the others are a few weeks behind, I think. 

The ones behind are in a bit of stasis  now, as there is not much new green growth.  I've heard that at some point you need to stop fertilizing due to green growth when it's cold, but not sure why.

I'm in NJ, so think freezing temps at night will be some time around late October. 

I'm in NJ as well. I only fert 1x in May with 10-10-10 / lime. I would stop now so new growth is not at risk of an early freeze. Sad to say we are only about 60 days from frost &/or snow. At this point any new figs will not ripen. I would strip most of them down to 3 or 4 figs per branch and remove all leaves around the fruit to pick up as much sunlight as possible. You have about 4 weeks to make it happen. Good luck!

i'm not sure I understand...the fruit needs sunlight to ripen?   I thought that the leaves use the sunlight to process energy and the fruit uses that energy to ripen.  Thus you can wrap the figs in newspaper to protect from birds, but they still ripen. 

Are you suggesting that trimming the leaves back will make the tree concentrate it's energy on producing fruit rather than maintaining the leaves?

Yes. Remove the figs that you know will not have time to ripen. Then remove the leaves around the figs that might. This will leave you with 1/2 the leaves you had before. That is more than enough to ripen the remaining figs.