It really is best for your garden’s long-term health to resist the urge to work the soil when it is still wet. Whether you use a tiller, plow or just a garden spade, working wet soil can badly compact soil, and the negative effects will last for many years.
Working wet soil will pack soil particles tightly, leaving less room for water and air to penetrate. Compacted soil also makes it more difficult for plant roots and gardening equipment to move through the soil. The compression forms tight clumps of soil that become hard as rocks upon drying and are difficult to break up. In addition to making it difficult for plants to grow, compacted soils also tend to drain more slowly, in turn delaying the ability to work the soil after the next rainfall.
Once compacted, it will take many years to rebuild a healthy soil structure, requiring annual applications of organic matter, such composted plant and animal wastes or perhaps growing a green manure crop, such as annual rye or winter wheat.
The best course of action is to prevent compaction in the first place. To determine whether your garden’s soil is dry enough to work, dig a trowel full of soil and squeeze it in your hand. Soil that crumbles through your fingers when squeezed is ready to garden. If, however, the soil forms a muddy ball, give the soil another few days to dry, and sample again later.