I had no prob tipping the helper for the light and in fact, also tipped the main guy. He wanted us to watch so we'd learn and even quizzed as he went along to make sure we knew. That was what got me started doing my own electrical. Well worth the price.
I always offer a cooler full of ice water, sodas, etc. and offer snacks too. I want my guys to feel appreciated. I really don't have extra dough to throw around. What would you give $-wise anyway? It's certainly not 15-20% of the bill like a restaurant.