Every so often they will have to go and top up their happiness - wells, churches, graveyards and taverns provide happiness so I tend to put a well near groups of crop fields as they take little space. There are other things that can also help keep workers from wandering too far from their work place. When leaving them assigned the game does a decent job of having them already close to their work place come early spring. Manually having to re-assign them at early spring could end up with them being halfway across the map so they don't start sowing at the earlist oppurtunity.
There's no point manually re-assigning them to other jobs in the winter, the game automatically makes any idle worker act as a labourer until needed in their assigned roll. Also, leave farmers assigned all year round. And educated workers are about 20-25% more productive than uneducated workers. As mentioned, have houses and barns (with available space) close by.
