Unless you have grease zerks on the spindles then the bearing will be a semi sealed type that don't require further lubrication. I say semi sealed because there is some grease exchange from outside in causes where they have used seal bearings, and greaseable spindles,. the problem is most people over grease the bearings, and blow the seals out, which is worse than not greasing them at all.
If you check the owners manual there is generally a bearing replacement schedule as part of routine maintanance.
Great Dane ( defunct commercial brand ) recommended replacing the spindle bearings every year,'
They go around 3-4 years in the mowers I service.
Spindle bearings take a beating and should be tapered rollers.
In fact some of the really good mowers did fit tapered rollers, but they are expensive to fit and require a modicum of inteligence to maintain.
Thus deep groove balls which are again a second best alternative but very cheap.