The reason why it's important to grease mower blade bearings and roller bearings after wash down is because water can penetrate easily into the bearing housing and cause rust and corrosion if it's not properly lubricated.
When the mower blade bearings and roller bearings are not properly greased or lubricated it can lead to bearing failure and premature wear.
The moisture from the water that gets into the bearing housings can cause the bearings to rust as well and seize.
The grease acts as a barrier to protect the mower blade bearings and roller bearings after washing down as well.
If your mower deck spindles have grease zerks it's best to grease the mower blade bearings every month or so to ensure it has enough grease.
If you have time cleaning your mower deck after every use will prevent build-up over the long run.
However if you are cleaning your mower 2 or 3 times throughout the mowing season, you'll keep it in good shape.