I have close to 6000 miles on my Dahon D7 and I'm still using the original brake pads on the front tire. I just keep shortening the brake cable to get the pads closer to the rim. The pads themselves don't seem fully worn down yet.

Should I still go ahead and replace them or if it ain't broke don't fix it?

Many brake pads have a wear line to let you know when they are due for replacement which is far better than hearing metal on metal...

If they are wearing evenly and still providing good stopping power, you're good.

You could touch up the brake pads with some sandpaper as they do become glazed and this may improve your braking.

yes. that's right. sand it and you'll be ok...