Overgreasing causes the grease in the housing to churn like in a blender. This causes excess heat and premature seal and bearing failure. There needs to be room for the grease to move without being in constant churning motion. Once you've greased a tapered roller or straight roller bearing you should be good for numerous times of use. Compare this to you car wheel bearings, many are NEVER greased or only when a brake job is performed. My recommendation is to grease your mower deck spindle bearings 1 to 2 times per season. Wheels can be greased much more frequently because they don't rotate at the same velocity or generally have the same sealing capability.