My general rule of thumb - put into it no more than you want to get out of it.
In other words, if the tractor's making you money - the decision is simple math.
If it's just a hobby, and/or you're not relying on the tractor for any kind of income - Just feel the bearings - if they're crunchy, replace them. If they roll smoothly - ignore them.
If you don't use it a lot and you keep it well oiled, even those old bearings will most likely outlast you.