Soil testing is recommended to give an accurate starting point for fertility decisions. The most accurate sampling time is just prior to seeding, but this isn’t always practical because time is needed to purchase and perhaps place the fertilizer before seeding.
The next best alternative for this time of year is early spring sampling done after the soil has thawed and is no longer saturated from snow melt. Some sampling can be done by well equipped agronomists prior to frost melt. The Canola Encyclopedia has details on soil sampling techniques.