This is a 'how long is a piece of string' question. Any seed needs watering if the soil is in danger of drying out - even Snow Peas. I'm not sure where the idea comes from that they don't need water until germinated. They might not need watering but it depends entirely on the weather. There isn't a rule to follow. Water when first sowing, so the seed can absorb moisture. Then water again if it doesn't rain in time to keep the soil just moist - that is the soil at a few inches depth, not necessarily the surface.