1. To make the sauce, in a small bowl or jar, mix the lime juice with 2 tablespoons fish sauce, brown sugar, water, minced chiles, and garlic, if using. Mix well, then taste, adding more fish sauce, if desired.
(The sauce can be made up to one day ahead, and left at room temperature. If using garlic, add that shortly before serving.)
2. To make the salads, bring a large pot of water to a boil. Add the rice noodles, reduce the heat to a simmer and cook until just tender, 3 to 4 minutes. Drain in a colander and immediately rinse the noodles very well with cold water. Shake the colander to remove excess water, then spread the noodles out on a clean kitchen towel to cool.
3. Divide the noodles into two or three serving bowls. Top each with carrots, cucumbers, and meat or tofu. Coarsely chop, or tear, the mint, coriander, or basil into pieces and strew the herbs over the tops of the salads.
4. If serving with spring rolls, fry the rolls, then slice the warm rolls into bite-size pieces and arrange them around the noodles. Add fried shallots, if using, then serve with sauce on the side, letting people help themselves.