When to add salt to dosa batter

Salt serves two primary purposes in baking:

1. To regulate yeast
   • Salt kills yeast. The addition of salt to a yeast leavened dough prevents the little beasties from growing completely out of control.
2. To enhance and mask specific flavors
   • Salt is almost a universal flavor enhancer. Virtually anything that tastes good, will taste better with salt. What typically comes as a surprise to people is that this holds true with sweet things too, particularly chocolate. The addition of even a tiny bit of salt can make a sweet dish significantly sweeter. It also serves to mask the taste of raw flour.

Update

Also according to Progressive Baker salt affects the strength and shelf life of baked goods.

For rice it depends.  Cooking any starch in water will first cause the starch granules to swell and eventually tangle up with each other (the gelatinization).  Dissolving sugars or salts in the water slows down the process by raising the temperature the swelling starts.  While few prefer pasta as a stuck blob of strands, the same is not the case for rice.  I like my Basmati loose, but my risotto and sushi sticky, so salt may be required for Basmati and optional for Arborio.

There are many techniques for controlling the starch for rice.  To control the starch gelatinization of rice:

1. cook it like pasta with lots of water, then drain; or 
2. par cook it

Method 1 won't avoid the grain surface starch gelatinization, but it will help with stickiness (you may oil coat it after draining).  Method 2 delays the starch release allowing you to finish a risotto in seven minutes. Cool for parties or for restaurants.