I often sow my seeds closer than the recommended space. I call it intensive growing method. The reasoning behind that is for home gardeners we are not looking for growing uniform crops. As a matter of fact, we want to stretch our harvest as long as we can for obvious reasons. Sowing seeds close will allow you to do just that.
When the veggies are growing closer together, they will not all mature at the same time. The spots where they are grow close, they will not grow as fast. That gives you the control to a certain extend of where and when you want them to grow. In any circumstance, don't make them too close together, or they will not grow properly. In fact, the key to success is to thin the plants at the right time and places. And enjoy the thinning as the garden grows.
Veggies suitable for growing this way include: radishes, baby bok choys, beets (enjoy the beet greens in early stage), lettuce Chinese Broccoli (Gai Lan- New Dimension Seed). Watering and fertilizing is also vitally important if you what to grow your veggies intensively. Fruit bearing veggies are usually not good candidates.